Semiconductor device

ABSTRACT

A structure is employed in which a first protective insulating layer; an oxide semiconductor layer over the first protective insulating layer; a source electrode and a drain electrode that are electrically connected to the oxide semiconductor layer; a gate insulating layer that is over the source electrode and the drain electrode and overlaps with the oxide semiconductor layer; a gate electrode that overlaps with the oxide semiconductor layer with the gate insulating layer provided therebetween; and a second protective insulating layer that covers the source electrode, the drain electrode, and the gate electrode are included. Furthermore, the first protective insulating layer and the second protective insulating layer each include an aluminum oxide film that includes an oxygen-excess region, and are in contact with each other in a region where the source electrode, the drain electrode, and the gate electrode are not provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/278,234, filed May 15, 2014, now allowed, which claims the benefit offoreign priority applications filed in Japan as Serial No. 2013-106223on May 20, 2013 and Serial No. 2013-106253 on May 20, 2013, all of whichare incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed in this specification and the like relates to asemiconductor device and a method for manufacturing the semiconductordevice.

In this specification and the like, a semiconductor device means alltypes of devices that can function by utilizing semiconductorcharacteristics. A transistor, a semiconductor circuit, an arithmeticunit, a memory device, an imaging device, an electro-optical device, apower generation device (e.g., a thin film solar cell and an organicthin film solar cell), and an electronic device are each an embodimentof the semiconductor device.

2. Description of the Related Art

Attention has been focused on a technique for formation of a transistorusing a semiconductor thin film formed over a substrate having aninsulating surface. The transistor is applied to a wide range ofelectronic devices such as an integrated circuit (IC) or an imagedisplay device (also simply referred to as a display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor has been attracting attention.

For example, a technique for formation of a transistor using zinc oxideor an In—Ga—Zn-based oxide semiconductor as an oxide semiconductor isdisclosed (see Patent Documents 1 and 2).

In addition, a technique in which oxide semiconductor layers withdifferent electron affinities (or conduction band minimum states) arestacked to increase the carrier mobility of a transistor is disclosed(see Patent Documents 3 and 4).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055-   [Patent Document 3] Japanese Published Patent Application No.    2011-124360-   [Patent Document 4] Japanese Published Patent Application No.    2011-138934

SUMMARY OF THE INVENTION

Improvement in reliability is important for commercialization ofsemiconductor devices including transistors that use an oxidesemiconductor. Variation and decrease in electrical characteristics of asemiconductor device are particularly a factor of reduction in thereliability thereof.

In view of the above problem, an object of one embodiment of the presentinvention is to provide a semiconductor device using an oxidesemiconductor and having high reliability.

Miniaturization of a transistor is necessary for high-speed operation,low power consumption, price reduction, high integration, or the like ofthe transistor.

Thus, another object of one embodiment of the present invention is toprovide a semiconductor device using an oxide semiconductor that isminiaturized and has favorable electrical characteristics.

Note that the descriptions of these objects do not disturb the existenceof other objects. Note that in one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

A semiconductor device of one embodiment of the present inventionincludes a first protective insulating layer; an oxide semiconductorlayer over the first protective insulating layer; a source electrode anda drain electrode that are electrically connected to the oxidesemiconductor layer; a gate insulating layer that is over the sourceelectrode and the drain electrode and overlaps with the oxidesemiconductor layer; a gate electrode that overlaps with the oxidesemiconductor layer with the gate insulating layer providedtherebetween; and a second protective insulating layer that covers thesource electrode, the drain electrode, and the gate electrode. The firstprotective insulating layer and the second protective insulating layereach include an aluminum oxide film that includes an oxygen-excessregion, and are in contact with each other in a region where the sourceelectrode, the drain electrode, and the gate electrode are not provided.

The gate electrode preferably covers a top surface and side surfaces ofthe oxide semiconductor layer with the gate insulating layer providedtherebetween.

The thickness of the oxide semiconductor layer is preferably 0.1 timesor more and 10 times or less as large as the channel width.

The semiconductor device may further include a first oxide layer that isbetween the first protective insulating layer and the oxidesemiconductor layer and contains at least one metal element contained inthe oxide semiconductor layer, and a second oxide layer that is betweenthe oxide semiconductor layer and the gate insulating layer and containsat least one metal element contained in the oxide semiconductor layer.Here, a conduction band minimum of the first oxide layer and aconduction band minimum of the second oxide layer are each closer to avacuum level than a conduction band minimum of the oxide semiconductorlayer. A difference between energy at the conduction band minimum of thefirst oxide layer and energy at the conduction band minimum of the oxidesemiconductor layer is 0.05 eV or higher and 2 eV or lower. A differencebetween energy at the conduction band minimum of the second oxide layerand the energy at the conduction band minimum of the oxide semiconductorlayer is 0.05 eV or higher and 2 eV or lower.

A top surface of the second oxide layer may be in contact with anundersurface of the source electrode, an undersurface of the drainelectrode, and an undersurface of the gate insulating layer.

Alternatively, an undersurface of the second oxide layer may be incontact with a top surface of the source electrode, a top surface of thedrain electrode, and a top surface and side surfaces of the oxidesemiconductor layer in a region where the source electrode and the drainelectrode are not provided.

A semiconductor device of another embodiment of the present inventionincludes an insulating layer including a groove; a first protectiveinsulating layer that covers a side surface and a bottom surface of thegroove; an oxide semiconductor layer that is over the first protectiveinsulating layer and fills the groove; a source electrode and a drainelectrode that are electrically connected to the oxide semiconductorlayer; a gate insulating layer that is over the source electrode and thedrain electrode and overlaps with the oxide semiconductor layer; a gateelectrode that overlaps with the oxide semiconductor layer with the gateinsulating layer provided therebetween; and a second protectiveinsulating layer that covers the source electrode, the drain electrode,and the gate electrode. Furthermore, the first protective insulatinglayer and the second protective insulating layer each include analuminum oxide film that includes an oxygen-excess region, and are incontact with each other in a region where the source electrode, thedrain electrode, and the gate electrode are not provided.

One embodiment of the present invention makes it possible to provide asemiconductor device using an oxide semiconductor and having highreliability.

One embodiment of the present invention makes it possible to provide asemiconductor device using an oxide semiconductor that is miniaturizedand has favorable electrical characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C illustrate a structural example of a semiconductor deviceof an embodiment.

FIGS. 2A to 2E illustrate an example of a method for manufacturing thesemiconductor device of the embodiment.

FIGS. 3A and 3B illustrate a structural example of a semiconductordevice of an embodiment.

FIG. 4 illustrates a structural example of a semiconductor device of anembodiment.

FIGS. 5A to 5C illustrate a structural example of a semiconductor deviceof an embodiment.

FIGS. 6A to 6E illustrate an example of a method for manufacturing thesemiconductor device of the embodiment.

FIGS. 7A and 7B illustrate a structural example of a semiconductordevice of an embodiment.

FIG. 8 illustrates a structural example of a semiconductor device of anembodiment.

FIGS. 9A to 9D illustrate a structural example of a semiconductor deviceof an embodiment and circuit diagrams thereof.

FIGS. 10A and 10B illustrate an example of a structure of asemiconductor device of an embodiment.

FIGS. 11A and 11B each show a band diagram of an embodiment.

FIGS. 12A and 12B illustrate a structural example of a semiconductordevice of an embodiment.

FIGS. 13A to 13D illustrate structural examples of a semiconductordevice of an embodiment.

FIGS. 14A to 14D illustrate structural examples of a semiconductordevice of an embodiment.

FIG. 15 illustrates a structural example of a semiconductor device of anembodiment.

FIGS. 16A to 16D illustrate structural examples and circuit diagrams ofa semiconductor device of an embodiment.

FIGS. 17A and 17B illustrate structural examples of a semiconductordevice of an embodiment.

FIG. 18 is an equivalent circuit diagram of a semiconductor device of anembodiment.

FIG. 19 is a circuit diagram of a semiconductor device of an embodiment.

FIG. 20 is a block diagram of a semiconductor device of an embodiment.

FIG. 21 is a circuit diagram of a memory device of an embodiment.

FIGS. 22A to 22C illustrate electronic devices of an embodiment.

FIGS. 23A to 23C are cross-sectional TEM images and a local Fouriertransform image of an oxide semiconductor.

FIGS. 24A and 24B show nanobeam electron diffraction patterns of oxidesemiconductor films and FIGS. 24C and 24D illustrate an example of atransmission electron diffraction measurement apparatus.

FIG. 25A shows an example of structural analysis by transmissionelectron diffraction measurement and FIGS. 25B and 25C show planar TEMimages.

FIGS. 26A and 26B illustrate a structural example of a semiconductordevice of an embodiment.

FIGS. 27A and 27B illustrate a structural example of a semiconductordevice of an embodiment.

FIGS. 28A and 28B illustrate a structural example of a semiconductordevice of an embodiment.

FIGS. 29A to 29D illustrate structural examples of a semiconductordevice of an embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to drawings. Notethat the present invention is not limited to the description below, andit is easily understood by those skilled in the art that the mode anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Therefore, the present inventionshould not be interpreted as being limited to the following descriptionof the embodiments.

Note that in the structures of the invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description of suchportions is not repeated. The same hatching pattern is applied toportions having similar functions, and the portions are not especiallydenoted by reference numerals in some cases.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such a scale.

Note that in this specification and the like, ordinal numbers such as“first”, “second”, and the like are used in order to avoid confusionamong components and do not limit the number.

A transistor is a kind of semiconductor elements and can achieveamplification of current or voltage, switching operation for controllingconduction or non-conduction, or the like. A transistor in thisspecification includes an insulated-gate field effect transistor (IGFET)and a thin film transistor (TFT).

Embodiment 1

In this embodiment, a structure and a manufacturing method of atransistor, which is an example of a semiconductor device of oneembodiment of the present invention, are described with reference todrawings.

In the case of manufacturing a transistor using an oxide semiconductor,an oxygen vacancy can be given as a carrier supply source of the oxidesemiconductor. A large number of oxygen vacancies in an oxidesemiconductor including a channel formation region of a transistor leadto generation of electrons in the channel formation region, which causesdefects in electrical characteristics; for example, the transistorbecomes normally-on, leakage current increases, or threshold voltage isshifted due to stress application.

In an oxide semiconductor layer, hydrogen, silicon, nitrogen, carbon,and metal elements which are not the main components are impurities. Forexample, some hydrogen forms a donor level in the oxide semiconductorlayer, which results in an increase in carrier density.

For these reasons, to obtain stable electrical characteristics of asemiconductor device using an oxide semiconductor, some measures need tobe taken to reduce oxygen vacancies by supplying an adequate amount ofoxygen to the oxide semiconductor layer and to reduce the concentrationsof impurities such as hydrogen.

In view of the above, in a semiconductor device of one embodiment of thepresent invention, a protective insulating layer including an aluminumoxide film with an oxygen-excess region is provided to surround an oxidesemiconductor layer, so that oxygen is supplied from the protectiveinsulating layer to a channel formation region; thus, oxygen vacanciesthat might be formed in the channel formation region are filled.Furthermore, the protective insulating layer inhibits release of oxygenfrom the oxide semiconductor layer, which makes it possible to inhibitformation of oxygen vacancies.

In one embodiment of the present invention, an insulating layerincluding an aluminum oxide film that contains excess oxygen is used asa protective insulating layer for supplying oxygen to a channelformation region. Here, the term “excess oxygen” refers to, for example,oxygen contained in excess of the stoichiometric composition or oxygenthat might be released by heating at a temperature lower than or equalto that applied in the manufacturing process of a semiconductor device.As the aluminum oxide film containing excess oxygen, an AlOx (x isgreater than 3/2) film can be used, for example. Excess oxygen containedin the aluminum oxide film can be released by heating and supplied to anoxide semiconductor layer. Therefore, by providing an insulating layerincluding the aluminum oxide film over and under the oxide semiconductorlayer, oxygen can be effectively supplied to the channel formationregion.

Note that the aluminum oxide film containing excess oxygen can be formedby a sputtering method or the like in an atmosphere containing oxygen,for example.

The aluminum oxide film is an insulating layer that is less permeable tooxygen and hydrogen than an insulating layer such as a silicon oxidefilm or a silicon oxynitride film or the oxide semiconductor layer is.In other words, the aluminum oxide film is an insulating layer with abarrier property to oxygen and hydrogen. Thus, with an insulating layerincluding the aluminum oxide film, oxygen vacancies formed due torelease of oxygen can be reduced in a region surrounded by theinsulating layer and mixing of hydrogen or a hydrogen compound into theregion can be inhibited.

In one embodiment of the present invention, in a region where the oxidesemiconductor layer and source and drain electrodes electricallyconnected to the oxide semiconductor layer are not provided, protectiveinsulating layers provided over and under the oxide semiconductor layerare in contact with each other. That is, in the semiconductor device ofone embodiment of the present invention, aluminum oxide films areprovided to wrap the oxide semiconductor layer. Such a structure makesit possible to supply oxygen and to reduce mixing of impurities such ashydrogen and/or release of oxygen into/from a side surface of the oxidesemiconductor layer in addition to interfaces with the oxidesemiconductor layer on the front channel side and the back channel side.Consequently, a change in the electrical characteristics of a transistorin which a channel is formed in the oxide semiconductor layer issuppressed, which enables a semiconductor device to have highreliability.

By reducing oxygen vacancies in the channel formation region, thesemiconductor device of one embodiment of the present invention can havefavorable electrical characteristics and can be a highly reliablesemiconductor device in which a change in the electrical characteristicsis suppressed.

Effects of a structure of one embodiment of the present invention can bedescribed, for example, as follow.

In a semiconductor device of one embodiment of the present invention,insulating layers each including an aluminum oxide film that containsexcess oxygen are provided to wrap an oxide semiconductor layer. Theexcess oxygen contained in the aluminum oxide film is supplied to theoxide semiconductor layer including a channel through heat treatment inthe manufacturing process of the semiconductor device. In addition,since the aluminum oxide film has a barrier property to oxygen andhydrogen, release of oxygen from the oxide semiconductor layer wrappedwith the insulating layers each including the aluminum oxide film andmixing of impurities such as hydrogen into the oxide semiconductor layercan be reduced. An oxide semiconductor layer which is supplied with anadequate amount of oxygen and into which impurities such as hydrogen areinhibited from being mixed is a highly purified intrinsic oxidesemiconductor layer.

Furthermore, in the semiconductor device, a gate electrode overlappingwith the oxide semiconductor layer with a gate insulating layer providedtherebetween is preferably provided to overlap with side surfaces and atop surface of the channel formation region in the oxide semiconductorlayer. With such a structure, an electric field is applied to the oxidesemiconductor layer in a direction perpendicular to the side surfacesand a direction perpendicular to the top surface, which makes itpossible to favorably control the threshold voltage of the transistorand improve the subthreshold swing (also referred to as S value)thereof.

Miniaturization of a transistor is necessary for a semiconductor deviceto have high density (a high degree of integration). On the other hand,it is known that miniaturization of a transistor might causedeterioration of the electrical characteristics of the transistor.

For example, it is known that a transistor using silicon with a shortchannel length produces short-channel effects such as deterioration ofthe subthreshold swing (S value) and a shift in the threshold voltage.

In contrast, in a transistor using an oxide semiconductor that is anaccumulation transistor in which electrons are majority carriers,drain-induced barrier lowering (DIBL) in a short channel is less likelyto occur than in an inversion transistor such as the transistor usingsilicon. In other words, the transistor using an oxide semiconductor isresistant to a short-channel effect.

In addition, a transistor with a short channel width might reduce anon-state current. For the purpose of increasing the on-state current,there is a method in which the thickness of an active layer is increasedso that a channel is formed on side surfaces of the active layer.However, an increase in a surface area where a channel is formedincreases scattering of carriers at the interface between a channelformation region and a gate insulating layer; therefore, it is not easyto increase the on-state current sufficiently.

However, in the transistor of one embodiment of the present invention,the insulating layers each including the aluminum oxide film thatcontains excess oxygen are included to wrap the oxide semiconductorlayer in which the channel is formed; thus, excess oxygen contained inthe aluminum oxide film can be supplied to the oxide semiconductor layerand release of oxygen and mixing of impurities such as hydrogenfrom/into the oxide semiconductor layer can be reduced. Because anoxygen vacancy and hydrogen are factors in generating carriers in theoxide semiconductor layer, providing the aluminum oxide film containingexcess oxygen can reduce scattering of carriers that might occur at aninterface with the oxide semiconductor layer in which the channel isformed.

Accordingly, even when the channel width is decreased, an on-statecurrent can be sufficiently increased by increasing the thickness of theoxide semiconductor layer to increase a surface area of the oxidesemiconductor layer that overlaps with the gate electrode. Tosufficiently apply an electric field from the gate electrode in the sidesurface direction of the oxide semiconductor layer, the thickness of theoxide semiconductor layer is preferably greater than or equal to thechannel width.

Furthermore, it is effective to provide an oxide layer containing atleast one metal element in the oxide semiconductor layer in contact withthe oxide semiconductor layer, in which case the above-describedscattering of carriers can be further reduced.

Note that when the channel length and channel width of the transistorare extremely small, end surfaces of a wiring, a semiconductor layer,and the like which are processed with the use of a resist mask arerounded (curved) in some cases. In the case of forming a thin insulatinglayer (e.g., gate insulating layer) to cover the oxide semiconductorlayer with a large thickness, shape defects are caused by poor coveragewith the insulating layer, so that stable electrical characteristicscannot be obtained in some cases. However, the oxide semiconductor layerwith a curved end surface can increase the coverage with the insulatinglayer provided over the oxide semiconductor layer, which is preferable.

In addition, some hydrogen in the oxide semiconductor layer is trappedby oxygen vacancies and thus the oxide semiconductor layer has n-typeconductivity; therefore, the Fermi level (Ef) gets near the conductionband minimum (Ec). Accordingly, with the oxide semiconductor layercontaining a large amount of hydrogen, the electrical characteristicsmight be changed but an increase in the field-effect mobility of thetransistor is expected. In the case where the oxide semiconductor layeris made intrinsic or substantially intrinsic, the Fermi energy of theoxide semiconductor layer is aligned with or gets near the mid gap(energy in the middle of the energy gap of the oxide semiconductorlayer) as much as possible. In this case, the field-effect mobilitymight be reduced because of a decrease in the number of carrierscontained in the oxide semiconductor layer.

-   However, in the transistor of one embodiment of the present    invention, a gate electric field is applied to the oxide    semiconductor layer in the side surface direction in addition to the    perpendicular direction. That is, the gate electric field is applied    entirely to the oxide semiconductor layer, so that current flows    through the bulk of the oxide semiconductor layer. Consequently, a    change in the electrical characteristics can be suppressed owing to    the highly purified intrinsic oxide semiconductor layer and the    field-effect mobility of the transistor can be increased.

More specifically, the following structures can be employed for example.

Structural Example 1

FIG. 1A is a schematic top view of a transistor 100 described as anexample in this structural example. FIG. 1B is a schematiccross-sectional view taken along line A-B in FIG. 1A, and FIG. 1C is aschematic cross-sectional view taken along line C-D in FIG. 1A. Notethat some components are not illustrated in FIG. 1A for clarification.

The transistor 100 is provided over a substrate 101 and includes anisland-shaped semiconductor layer 102, a pair of electrodes 103electrically connected to the semiconductor layer 102, a gate insulatinglayer 104 that is over the pair of electrodes 103 and overlaps with thesemiconductor layer 102, and a gate electrode 105 that is over the gateinsulating layer 104 and overlaps with the semiconductor layer 102.

A first protective insulating layer 111 is provided between thesubstrate 101 and the semiconductor layer 102. A second protectiveinsulating layer 112 is provided over the pair of electrodes 103 and thegate electrode 105. Furthermore, the first protective insulating layer111 and the second protective insulating layer 112 are in contact witheach other in a region where the pair of electrodes 103 and the gateelectrode 105 are not provided.

The semiconductor layer 102 contains an oxide semiconductor. Thesemiconductor layer 102 preferably contains at least indium (In) or zinc(Zn). Alternatively, the semiconductor layer 102 preferably containsboth In and Zn, more preferably contains an In—M—Zn-based oxide (M is ametal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf).

The pair of electrodes 103 serves as a source electrode and a drainelectrode of the transistor 100. In FIG. 1B, the pair of electrodes 103is provided in contact with a top surface and side surfaces of thesemiconductor layer 102.

The gate electrode 105 is provided to surround the top surface and theside surfaces of the semiconductor layer 102 with the gate insulatinglayer 104 provided therebetween.

Here, the channel length (L) of the transistor is a distance between thesource and the drain which face each other. The channel width (W) of thetransistor is the width of the semiconductor layer in a directionorthogonal to the channel length direction. Note that in the transistor,depending on the shapes of the source electrode, the drain electrode,the gate electrode, and the semiconductor layer, the channel length andthe channel width are not uniform in some cases. In such a case, theaverage channel length or the minimum channel length can be used as thechannel length of the transistor, and the average channel width or theminimum channel width can be used as the channel width of thetransistor.

The gate electrode 105 is provided to surround the side surfaces of thesemiconductor layer 102; therefore, the side surfaces of thesemiconductor layer 102 can serve as a channel formation region. In thiscase, the thickness of the semiconductor layer 102 is preferably 0.05times or more and 20 times or less, more preferably 0.1 times or moreand 10 times or less as large as the channel width of the semiconductorlayer 102. With such a shape, a decrease in an on-state current can besuppressed even when the channel width is reduced, resulting in afurther miniaturized transistor capable of operating at high speed.

A structure of such a transistor in which a gate electrode is providedto surround a top surface and side surfaces of a semiconductor layer anda channel formed in the vicinity of the side surfaces of thesemiconductor layer is actively used to increase an on-state current canbe referred to as a surrounded channel (S-channel) structure.

Each of the first protective insulating layer 111 and the secondprotective insulating layer 112 can be formed using an insulatingmaterial including an oxygen-excess region and having a function ofinhibiting diffusion of oxygen (also referred to as a property ofblocking oxygen). For example, a layer including an aluminum oxide filmcan be used as the first protective insulating layer 111 and the secondprotective insulating layer 112. Alternatively, a film containing aninsulating material that contains oxygen, such as aluminum oxide,aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide,yttrium oxynitride, hafnium oxide, hafnium oxynitride, oryttria-stabilized zirconia (YSZ) can be used.

As the insulating film including the oxygen-excess region, an oxideinsulating film containing oxygen in excess of the stoichiometriccomposition is preferably used, for example. Some oxygen is released byheating from the oxide insulating film containing oxygen in excess ofthe stoichiometric composition.

It is preferable to use an insulating material having an extremely lowhydrogen content for the first protective insulating layer 111 and thesecond protective insulating layer 112. For example, an insulatingmaterial including a region in which a hydrogen content measured bysecondary ion mass spectrometry (SIMS) is less than 5×10²¹ atoms/cm³,preferably less than 2×10²¹ atoms/cm³, more preferably less than 1×10²¹atoms/cm³ can be used.

Alternatively, a material in which silicon oxide is contained in any ofthe above-described oxides can be used as the insulating material forthe first protective insulating layer 111 and the second protectiveinsulating layer 112. For example, a material in which 0.1 wt % to 30 wt% (e.g., 5 wt % or 10 wt %) silicon oxide is contained in aluminum oxidecan be used. The use of the material containing 0.1 wt % to 30 wt %silicon oxide can increase the amount of oxygen released from by heatingwithout decreasing the property of blocking oxygen and can reduce thestress of a film.

[Components]

Components of the transistor 100 are described below.

[Semiconductor Layer]

An oxide semiconductor having a wider band gap and a lower carrierdensity than silicon is preferably used as the oxide semiconductorcontained in the semiconductor layer 102, in which case an off-statecurrent of the transistor can be reduced.

There is no particular limitation on the crystallinity of asemiconductor used for the semiconductor layer 102, and an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle crystal semiconductor, a semiconductor partly including crystalregions, or a semiconductor including crystal regions in the whole area)may be used. A semiconductor having crystallinity is preferably used forthe semiconductor layer 102, in which case deterioration of transistorcharacteristics can be reduced.

As the semiconductor layer 102, it is particularly preferable to use alayer including a plurality of crystal parts. In the plurality ofcrystal parts, c-axes are aligned substantially perpendicular to asurface on which the semiconductor layer 102 is formed (a top surface ofthe first protective insulating layer 111 in FIGS. 1B and 1C) or the topsurface of the semiconductor layer 102, and the crystal parts adjacentto each other have no grain boundary.

The use of such a material for the semiconductor layer 102 can suppressa change in the electrical characteristics, which allows the transistor100 to have high reliability.

The semiconductor layer 102 may have a single-layer structure or astacked-layer structure of two or more layers. In the case of thestacked-layer structure, a combination of two or more oxidesemiconductor films with different compositions may be used.

Note that details of a preferable mode and a method for forming an oxidesemiconductor applicable to the semiconductor layer 102 are described inan embodiment below.

[Substrate]

There is no particular limitation on the property of a material and thelike of the substrate 101 as long as the material has heat resistanceenough to withstand at least heat treatment in the process. For example,a glass substrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, an yttria-stabilized zirconia (YSZ) substrate, or the likemay be used as the substrate 101. Alternatively, a single crystalsemiconductor substrate or a polycrystalline semiconductor substratemade of silicon, silicon carbide, or the like, a compound semiconductorsubstrate made of silicon germanium or the like, an SOI substrate, orthe like can be used.

Still alternatively, any of the above-described semiconductor substratesor the SOI substrate provided with a semiconductor element may be usedas the substrate 101. In this case, the transistor 100 is formed overthe substrate 101 with an interlayer insulating layer providedtherebetween. The transistor 100 in this case may have a structure inwhich at least one of the gate electrode 105 and the pair of electrodes103 is electrically connected to the above semiconductor element by aconnection electrode embedded in the interlayer insulating layer.Forming the transistor 100 over the semiconductor element with theinterlayer insulating layer interposed therebetween can suppress anincrease in area due to the formation of the transistor 100.

[Gate Electrode]

The gate electrode 105 can be formed using a metal selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these metals as a component; analloy containing any of these metals in combination; or the like.Alternatively, one or both of manganese and zirconium may be used. Stillalternatively, a semiconductor typified by polycrystalline silicon dopedwith an impurity element such as phosphorus, or a silicide such asnickel silicide may be used. Furthermore, the gate electrode 105 mayhave a single-layer structure or a stacked-layer structure of two ormore layers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order, and the like can be given. Alternatively, an alloy filmor a nitride film which contains aluminum and one or more metalsselected from titanium, tantalum, tungsten, molybdenum, chromium,neodymium, and scandium may be used.

Still alternatively, the gate electrode 105 can also be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added. It is also possible to have astacked-layer structure formed using the above light-transmittingconductive material and the above metal.

An In—Ga—Zn-based oxynitride semiconductor film, an In—Sn-basedoxynitride semiconductor film, an In—Ga-based oxynitride semiconductorfilm, an In—Zn-based oxynitride semiconductor film, a Sn-basedoxynitride semiconductor film, an In-based oxynitride semiconductorfilm, a film of metal nitride (such as InN or ZnN), or the like may beprovided between the gate electrode 105 and the gate insulating layer104. These films each have a work function of 5 eV or higher, preferably5.5 eV or higher, which is higher than the electron affinity of an oxidesemiconductor; thus, the threshold voltage of a transistor including theoxide semiconductor can be shifted in the positive direction.Accordingly, a switching element having what is called normally-offcharacteristics can be obtained. For example, as an In—Ga—Zn-basedoxynitride semiconductor film, an In—Ga—Zn-based oxynitridesemiconductor film having a higher nitrogen concentration than at leastthe semiconductor layer 102, specifically an In—Ga—Zn-based oxynitridesemiconductor film having a nitrogen concentration higher than or equalto 7 at. %, is used.

[Gate Insulating Layer]

The gate insulating layer 104 can be formed to have a single-layerstructure or a stacked-layer structure using, for example, one or moreof silicon oxide, silicon oxynitride, silicon nitride oxide, aluminumoxide, hafnium oxide, gallium oxide, Ga—Zn-based metal oxide, siliconnitride, and the like.

Alternatively, the gate insulating layer 104 may be formed using ahigh-k material such as hafnium silicate (HfSiO_(x)), hafnium silicateto which nitrogen is added (HfSi_(x)O_(y)N_(z)), hafnium aluminate towhich nitrogen is added (HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttriumoxide, in which case gate leakage current of the transistor can bereduced.

[Pair of Electrodes]

The pair of electrodes 103 can be formed to have a single-layerstructure or a stacked-layer structure using, as a conductive material,any of metals such as aluminum, titanium, chromium, nickel, copper,yttrium, zirconium, molybdenum, silver, tantalum, and tungsten, or analloy containing any of these metals as its main component. For example,a single-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over acopper film, a two-layer structure in which a titanium film is stackedover a tungsten film, a two-layer structure in which a copper film isstacked over a copper-magnesium-aluminum alloy film, a three-layerstructure in which a titanium film or a titanium nitride film, analuminum film or a copper film, and a titanium film or a titaniumnitride film are stacked in this order, a three-layer structure in whicha molybdenum film or a molybdenum nitride film, an aluminum film or acopper film, and a molybdenum film or a molybdenum nitride film arestacked in this order, and the like can be given. Note that atransparent conductive material containing indium oxide, tin oxide, orzinc oxide may be used.

The above is the description of the structural example of the transistor100 and each of the components.

Example of Manufacturing Method 1

An example of a method for manufacturing the transistor 100 illustratedin FIG. 1A to 1C is described below with reference to the drawings.FIGS. 2A to 2E are schematic cross-sectional views of steps in themanufacturing method described below as an example.

[Formation of First Protective Insulating Layer]

First, the first protective insulating layer 111 is formed over thesubstrate 101 (FIG. 2A).

The first protective insulating layer 111 can be foil led by asputtering method or the like in an atmosphere containing oxygen, forexample. Alternatively, the first protective insulating layer 111 may beformed by a chemical vapor deposition (CVD) method, a molecular beamepitaxy (MBE) method, an atomic layer deposition (ALD) method, a pulsedlaser deposition (PLD) method, or the like in an atmosphere containingoxygen.

In the case where an aluminum oxide film is used as the first protectiveinsulating layer 111, for example, the first protective insulating layer111 can be formed in an atmosphere containing oxygen with the use ofaluminum oxide as a sputtering target. Note that an inert gas such as arare gas may be contained in a deposition gas. Oxygen is preferablycontained in the deposition gas so that the flow rate of the oxygenaccounts for 20% or higher, preferably 30% or higher, more preferably40% or higher of that of the deposition gas. Note that although thealuminum oxide film may be formed by a reactive sputtering method withthe use of aluminum as a sputtering target, aluminum oxide is preferablyused as the sputtering target because oxygen can be further contained inthe film.

[Formation of Semiconductor Layer]

Next, a semiconductor film is formed over the first protectiveinsulating layer 111. A resist mask is formed over the semiconductorfilm by a photolithography method or the like and unnecessary portionsof the semiconductor film are etched. Then, the resist mask is removed.Thus, the island-shaped semiconductor layer 102 can be formed (FIG. 2B).

The semiconductor film can be formed by a sputtering method, a CVDmethod, a MBE method, an ALD method, a PLD method, or the like.Alternatively, a technique for formation of a thin film using a liquidmaterial, such as a sol-gel method, a spray method, or a mist method,can be used. The semiconductor film is preferably formed by a sputteringmethod. As the sputtering method, an RF sputtering method, a DCsputtering method, an AC sputtering method, or the like can be used. Inparticular, a DC sputtering method is preferably used because dustgenerated in the film formation can be reduced and the film thicknesscan be uniform.

Note that heat treatment may be performed after the formation of thesemiconductor film. The heat treatment may be performed at 250° C. orhigher and 650° C. or lower, preferably 300° C. or higher and 500° C. orlower in an inert gas atmosphere, in an atmosphere containing anoxidization gas at 10 ppm or more, or under reduced pressure.Alternatively, the heat treatment may be performed in such a manner thatheat treatment is performed in an inert gas atmosphere, and then anotherheat treatment is performed in an atmosphere containing an oxidizationgas at 10 ppm or more, in order to compensate desorbed oxygen. By theheat treatment, oxygen is supplied from the first protective insulatinglayer 111 to the semiconductor film (or the semiconductor layer 102),which enables a reduction in oxygen vacancies in the oxide semiconductorincluded in the semiconductor layer 102. Note that the heat treatmentmay be performed directly after the formation of the semiconductor filmor may be performed after the semiconductor film is processed into theisland-shaped semiconductor layer 102.

As light used to form the resist mask, light with an i-line (with awavelength of 365 nm), light with a g-line (with a wavelength of 436nm), light with an h-line (with a wavelength of 405 nm), or light inwhich the i-line, the g-line, and the h-line are mixed can be used.Alternatively, ultraviolet light, KrF laser light, ArF laser light, orthe like can be used. Exposure may be performed by liquid immersionexposure technique. As the light for the exposure, extreme ultra-violetlight (EUV) or X-rays may be used. Instead of the light for theexposure, an electron beam can be used. It is preferable to use EUV,X-rays, or an electron beam because extremely minute processing can beperformed. Note that in the case of performing exposure by scanning abeam such as an electron beam, a photomask is not needed.

Here, as illustrated in FIG. 2B, in etching the semiconductor film, thefirst protective insulating layer 111 might be partly etched to have asmall thickness in a region where the semiconductor layer 102 does notoverlap with the first protective insulating layer 111. When the topsurface of the first protective insulating layer 111 that is in thevicinity of the semiconductor layer 102 is positioned lower than anundersurface of the semiconductor layer 102, lower parts of the sidesurfaces of the semiconductor layer 102 can be surrounded by the gateelectrode 105 formed later. Consequently, the electric field from thegate electrode 105 is sufficiently applied to the lower parts of theside surfaces of the semiconductor layer 102, which makes it possible toincrease the on-state current of the transistor 100. As illustrated inFIGS. 26A and 26B, the first protective insulating layer 111 ispreferably partly etched in a manner similar to the above so that anundersurface of the gate electrode 105 is positioned lower than theundersurface of the semiconductor layer 102, in which case the on-statecurrent of the transistor 100 can be further increased.

The first protective insulating layer 111 is not etched in some casesdepending on the material of the first protective insulating layer 111and etching conditions of the semiconductor film. At this time, thecoverage with a film formed over the semiconductor layer 102 isincreased, which is preferable.

Furthermore, as illustrated in FIG. 2B, the semiconductor layer 102 ispreferably processed so that the top surface thereof has smooth curvededges. Such a shape tends to be obtained particularly in the case wherethe semiconductor layer 102 is finely processed. The semiconductor layer102 with such a shape is preferable because the coverage with a filmformed thereover is increased and thus variations and a change in theelectrical characteristics of the transistor 100 can be suppressed.

[Formation of Pair of Electrodes]

Next, a conductive film is formed over the first protective insulatinglayer 111 and the semiconductor layer 102. A resist mask is formed overthe conductive film by a photolithography method or the like andunnecessary portions of the conductive film are etched. Then, the resistmask is removed. Thus, the pair of electrodes 103 can be formed (FIG.2C).

The conductive film can be formed by a sputtering method, an evaporationmethod, a CVD method, or the like, for example.

Here, as illustrated in FIG. 2C, in some cases, an upper portion of thesemiconductor layer 102 is partly etched in the etching of theconductive film to reduce the thickness of a portion where the pair ofelectrodes 103 does not overlap with the semiconductor layer 102. Forthis reason, the semiconductor film serving as the semiconductor layer102 is preferably formed to have a large thickness in advance inconsideration of the etching depth.

Although not clearly illustrated in FIG. 2C, also in the etching of theconductive film, part of the first protective insulating layer 111 mightbe etched to have a small thickness in a manner similar to the above.

[Formation of Gate Insulating Layer and Gate Electrode]

Next, an insulating film is formed over the semiconductor layer 102, thepair of electrodes 103, and the first protective insulating layer 111.Next, a conductive film is formed over the insulating film. A resistmask is formed over the conductive film by a photolithography method orthe like and unnecessary portions of the conductive film and theinsulating films are etched. Then, the resist mask is removed. Thus, thegate electrode 105 and the gate insulating layer 104 can be formed (FIG.2D).

The insulating film serving as the gate insulating layer 104 can beformed by a sputtering method, a CVD method, an MBE method, an ALDmethod, a PLD method, or the like. In particular, it is preferable thatthe insulating film be formed by a CVD method, further preferably aplasma CVD method because coverage can be further improved.

The conductive film serving as the gate electrode 105 can be formed by asputtering method, an evaporation method, a CVD method, or the like, forexample.

Note that here, the gate insulating layer 104 is etched at the same timewhen the gate electrode 105 is formed, so that the gate insulating layer104 is processed to have a shape similar to that of the gate electrode105 when seen from above. However, the gate insulating layer 104 and thegate electrode 105 may be processed individually so that the gateinsulating layer 104 extends to the outside of the gate electrode 105.At this time, a multi-tone mask such as a gray-tone mask or a half-tonemask is preferably used as a light-exposure mask used in thephotolithography method or the like, in which case the manufacturingprocess can be simplified.

[Formation of Second Protective Insulating Layer]

Next, the second protective insulating layer 112 is formed over thefirst protective insulating layer 111, the pair of electrodes 103, thegate insulating layer 104, and the gate electrode 105 (FIG. 2E).

The second protective insulating layer 112 can be formed by a methodsimilar to that of the first protective insulating layer 111.

Here, the second protective insulating layer 112 is provided in contactwith the first protective insulating layer 111 in a region where thepair of electrodes 103 and the gate electrode 105 are not provided.Thus, the first protective insulating layer 111 and the secondprotective insulating layer 112 can surround the semiconductor layer102.

Through the above steps, the transistor 100 can be manufactured.

[Heat Treatment]

Heat treatment may be performed after the second protective insulatinglayer 112 is formed. Through the heat treatment, oxygen is supplied fromthe first protective insulating layer 111 and the second protectiveinsulating layer 112 to the semiconductor layer 102, whereby oxygenvacancies in the semiconductor layer 102 can be reduced. At this time,release of oxygen from the semiconductor layer 102 is inhibited by thefirst protective insulating layer 111 and the second protectiveinsulating layer 112, so that formation of oxygen vacancies in thesemiconductor layer 102 can be inhibited.

The above is the description of the example of the manufacturing processof the transistor 100.

Modification Examples of Structural Example 1

Structural examples of a transistor that are partly different from thestructure of the transistor described in the structural example 1 aredescribed below. Note that description of the portions already describedis omitted and only different portions are described in detail. Evenwhen positions and shapes of components are different from those in theabove example, the same reference numerals are used as long as thecomponents have the same functions as those in the above example, anddetailed description thereof is omitted in some cases.

Modification Example 1

FIGS. 3A and 3B are schematic cross-sectional views of a transistordescribed as an example below. Note that a schematic top view of thetransistor can be referred to FIG. 1A. The transistor illustrated inFIGS. 3A and 3B is different from that in FIGS. 1B and 1C mainly in thatan insulating layer 106 is provided between the semiconductor layer 102and the first protective insulating layer 111.

The insulating layer 106 provided under the semiconductor layer 102preferably contains an oxide insulating material that releases oxygenwhen heated. With the insulating layer 106 provided under thesemiconductor layer 102, due to heat applied in heat treatment or thelike in the manufacturing process of the transistor, more oxygen can besupplied to the semiconductor layer 102. Furthermore, with a structurein which the insulating layer 106, the semiconductor layer 102, and thelike are surrounded by the first protective insulating layer 111 and thesecond protective insulating layer 112, oxygen released from theinsulating layer 106 is inhibited from being diffused to the outside(the substrate 101 side or above the second protective insulating layer112), which makes it possible to supply oxygen to the semiconductorlayer 102 more effectively.

Although the insulating layer 106 can be provided to cover the topsurface of the first protective insulating layer 111, as illustrated inFIGS. 3A and 3B, the insulating layer 106 is preferably processed withthe same resist mask as the semiconductor layer 102 so that the shapesof the semiconductor layer 102 and the insulating layer 106 aresubstantially aligned with each other when seen from above. With such astructure, the first protective insulating layer 111 and the secondprotective insulating layer 112 are in contact with each other in theregion where the gate electrode 105 and the pair of electrodes 103 arenot provided, so that an oxygen diffusion path is blocked; thus, oxygencan be supplied to the semiconductor layer 102 effectively.

As the insulating layer 106, an oxide insulating film containing oxygenin excess of the stoichiometric composition is preferably used. Part ofoxygen is released by heating from the oxide insulating film containingoxygen in excess of the stoichiometric composition. The oxide insulatingfilm containing oxygen in excess of the stoichiometric composition is anoxide insulating film in which the amount of released oxygen convertedinto oxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³,preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in thermaldesorption spectroscopy (TDS) analysis.

In the case of forming a silicon oxide film or a silicon oxynitride filmas the insulating layer 106 by a plasma CVD method, a deposition gascontaining silicon and an oxidization gas are preferably used as asource gas. Typical examples of the deposition gas containing siliconinclude silane, disilane, trisilane, and silane fluoride. Examples ofthe oxidization gas include oxygen, ozone, dinitrogen monoxide, andnitrogen dioxide.

For example, a silicon oxide film or a silicon oxynitride film is formedin the following conditions: the substrate placed in a vacuum-evacuatedtreatment chamber of a plasma CVD apparatus is held at a temperaturehigher than or equal to 180° C. and lower than or equal to 260° C.,preferably higher than or equal to 200° C. and lower than or equal to240° C., to the treatment chamber is charged a source gas at a pressuregreater than or equal to 100 Pa and less than or equal to 250 Pa,preferably greater than or equal to 100 Pa and less than or equal to 200Pa, and high-frequency power higher than or equal to 0.17 W/cm² andlower than or equal to 0.5 W/cm², preferably higher than or equal to0.25 W/cm² and lower than or equal to 0.35 W/cm² is supplied to anelectrode provided in the treatment chamber.

As the film formation conditions, the high-frequency power having theabove power density is supplied to the treatment chamber having theabove pressure, whereby the decomposition efficiency of the source gasin plasma is increased, oxygen radicals are increased, and oxidation ofthe source gas is promoted; therefore, oxygen is contained in the oxideinsulating film containing oxygen in excess of the stoichiometriccomposition. However, in the case where the substrate temperature iswithin the above temperature range, the bond between silicon and oxygenis weak, and accordingly, part of oxygen is released by heating. Thus,it is possible to form an oxide insulating film which contains oxygen inexcess of the stoichiometric composition and from which part of oxygenis released by heating.

Modification Example 2

FIG. 4 illustrates an example of forming a capacitor 120 adjacent to thetransistor 100.

The capacitor 120 includes one of the pair of electrodes 103 of thetransistor 100, an electrode 125 formed by processing the conductivefilm also used for the gate electrode 105, and a dielectric layer 124between the one of the pair of electrodes 103 and the electrode 125 thatis formed by processing the insulating film also used for the gateinsulating layer 104.

Being formed by processing the films used in manufacturing thetransistor 100 in such a manner, the capacitor 120 can be manufacturedat the same time as the transistor 100 without increasing the number ofmanufacturing steps.

Note that although the one of the pair of electrodes 103 of thetransistor 100 is used as one electrode of the capacitor 120 in FIG. 4,one embodiment of the present invention is not limited to this. Anelectrode formed by processing the conductive film also used for thepair of electrodes 103 of the transistor 100 may be used as the oneelectrode of the capacitor 120. Alternatively, at least the gateelectrode 105 and the electrode 125 may be formed as one continuousfilm. Still alternatively, at least the gate insulating layer 104 andthe dielectric layer 124 may be formed as one continuous film.

Here, as a material of the insulating film used to form the gateinsulating layer 104 and the dielectric layer 124, a high dielectricconstant material such as aluminum oxide, hafnium oxide, zirconiumoxide, tantalum oxide, titanium oxide, strontium titanate, or bariumtitanate is preferably used. Alternatively, any of these materialscontaining a metal such as lanthanum, aluminum, yttrium, or tungsten ora material containing an oxide of any of these metals may be used. Stillalternatively, a stacked-layer structure of films containing any of theabove materials may be used.

As the insulating film, an oxide insulating film containing oxygen inexcess of the stoichiometric composition is preferably used. With theuse of such an insulating film, due to heat applied in heat treatment orthe like in the manufacturing process of the transistor, oxygen can besupplied from the gate insulating layer 104 to the semiconductor layer102.

The above is the description of the modification examples.

Structural Example 2

Structural examples of a transistor that are partly different from thestructure of the transistor described in the structural example 1 aredescribed below. Note that portions similar to those described above arenot described in some cases.

FIG. 5A is a schematic top view of a transistor 200 described as anexample in this structural example. FIG. 5B is a schematiccross-sectional view taken along line E-F in FIG. 5A, and FIG. 5C is aschematic cross-sectional view taken along line G-H in FIG. 5A. Notethat some components are not illustrated in FIG. 5A for clarification.

The transistor 200 includes an insulating layer 207 that is providedover a substrate 201 and includes a groove, a semiconductor layer 202that is provided over the groove to fill the groove, a pair ofelectrodes 203 that is provided over the semiconductor layer 202 andelectrically connected to the semiconductor layer 202, a gate insulatinglayer 204 that is over the pair of electrodes 203 and overlaps with thesemiconductor layer 202, and a gate electrode 205 that is over the gateinsulating layer 204 and overlaps with the semiconductor layer 202.

In addition, under the semiconductor layer 202, a first protectiveinsulating layer 211 is provided to cover side surfaces and a bottomsurface of the groove in the insulating layer 207. As illustrated inFIGS. 5B and 5C, the first protective insulating layer 211 is preferablyprovided to cover a top surface of the insulating layer 207 that is in aregion where the groove is not provided. A second protective insulatinglayer 212 is provided to cover the pair of electrodes 203 and the gateelectrode 205. Furthermore, the first protective insulating layer 211and the second protective insulating layer 212 are in contact with eachother in a region where the pair of electrodes 203 and the gateelectrode 205 are not provided.

For the semiconductor layer 202, the pair of electrodes 203, the gateinsulating layer 204, the gate electrode 205, and the like, materialssimilar to those of the semiconductor layer 102, the pair of electrodes103, the gate insulating layer 104, the gate electrode 105, and the likein the structural example 1 can be used. For the first protectiveinsulating layer 211 and the second protective insulating layer 212,materials similar to those of the first protective insulating layer 111and the second protective insulating layer 112 in the structural example1 can be used.

The first protective insulating layer 211 is provided to cover the sidesurfaces and the bottom surface of the groove provided in the insulatinglayer 207, and the semiconductor layer 202 is provided to fill thegroove. The side surfaces and the undersurface of the semiconductorlayer 202 are surrounded by the first protective insulating layer 211.This makes it possible to inhibit diffusion of impurities such ashydrogen from the insulating layer 207 into the semiconductor layer 202and release of oxygen from the semiconductor layer 202 to the insulatinglayer 207.

Furthermore, the thickness of the semiconductor layer 202 can beincreased by adjusting the depth of the groove; therefore, the on-statecurrent of the transistor 200 and the withstanding voltage between asource and a drain can be easily increased. In the case where a thicksemiconductor layer is formed over a flat surface, a film formedthereover is difficult to cover the semiconductor layer, and the filmmight be divided or a region with low density might be formed in thefilm. In contrast, in this structural example, the semiconductor layer202 is provided to fill the groove, and the top surface of thesemiconductor layer 202 and the top surface of the first protectiveinsulating layer 211 are substantially equal to each other in height.For this reason, the thick semiconductor layer 202 can be formed withoutadversely affecting the coverage with the film formed over thesemiconductor layer 202.

The above is the description of the structural example of the transistor200.

Example of Manufacturing Method 2

An example of a method for manufacturing the transistor 200 illustratedin FIG. 5A to 5C is described below with reference to the drawings.FIGS. 6A to 6E are schematic cross-sectional views of steps in themanufacturing method described below as an example.

[Formation of Insulating Layer]

First, the insulating layer 207 is formed over the substrate 201.

The insulating layer 207 can be formed by a sputtering method, a CVDmethod, an evaporation method, or the like.

For the insulating layer 207, an insulating material such as siliconoxide, silicon oxynitride, silicon nitride, silicon nitride oxide,aluminum oxide, aluminum oxynitride, gallium oxide, gallium oxynitride,yttrium oxide, yttrium oxynitride, hafnium oxide, or hafnium oxynitridecan be used.

Alternatively, as the insulating layer 207, a stacked-layer structureincluding films of different insulating materials may be used. With theinsulating layer 207 having a stacked-layer structure, a lower layer inthe stacked-layer structure can serve as an etching stopper when thegroove is formed in a later step.

[Formation of Groove]

Next, a resist mask is formed over the insulating layer 207 by aphotolithography method or the like, and an upper portion of theinsulating layer 207 is removed. Then, the resist mask is removed. Thus,the groove can be formed in the insulating layer 207.

Here, as described above, a multi-layered structure including films ofdifferent materials is employed for the insulating layer 207, so thatetching can be easily performed. Furthermore, with the use of a lowerlayer in the multi-layered structure as an etching stopper, the groovecan have a flat bottom surface, which is preferable.

In the case of forming a groove with a great depth, the resist maskmight disappear during the etching. In such a case, a thin film isformed in advance using a material which is less likely to be etched inetching the insulating layer 207 (i.e., a material with which theetching selectivity of the insulating layer 207 to the thin film ishigh), and etched using the resist mask. Then, an upper portion of theinsulating layer 207 may be etched with the use of the thin film as ahard mask to form the groove. When the thin film used as the hard maskhas an insulating property, the hard mask may be left even after thegroove is formed.

[Formation of First Protective Insulating Layer]

Next, the first protective insulating layer 211 is formed over theinsulating layer 207 to cover the side surfaces and the bottom surfaceof the groove (FIG. 6A).

The first protective insulating layer 211 is formed in a manner similarto that of the first protective insulating layer 111 in the example ofthe manufacturing method 1.

[Formation of Semiconductor Layer]

Next, a semiconductor film is formed over the first protectiveinsulating layer 211. In the case of filling the groove with thesemiconductor film completely, a top surface of the semiconductor filmthat overlaps with the groove is preferably positioned higher than orequal to the top surface of the first protective insulating layer 211that does not overlap with the groove.

The semiconductor film can be formed in a manner similar to that in theexample of the manufacturing method 1.

Heat treatment may be performed after the formation of the semiconductorfilm. The heat treatment can be performed in a manner similar to that inthe example of the manufacturing method 1. By the heat treatment, oxygenis supplied from the first protective insulating layer 211 to thesemiconductor film (or the semiconductor layer 202), which enables areduction in oxygen vacancies in the oxide semiconductor included in thesemiconductor layer 202. Note that the heat treatment may be performeddirectly after the formation of the semiconductor film or may beperformed after the semiconductor film is processed into theisland-shaped semiconductor layer 202.

Next, planarization treatment is performed to process the semiconductorfilm and the first protective insulating layer 211 so that the topsurface of the semiconductor film is aligned with the top surface of thefirst protective insulating layer 211 that does not overlap with thegroove; thus, the island-shaped semiconductor layer 202 embedded in thegroove can be formed (FIG. 6B).

As the planarization treatment, etching treatment or polishing treatmentsuch as chemical mechanical polishing (CMP) may be performed.

Here, in the case where aluminum oxide or the like is used for the firstprotective insulating layer 211 and polishing treatment such as CMP isused as the planarization treatment, the first protective insulatinglayer 211 can serve as an etching stopper. Consequently, a decrease inthe thickness of the semiconductor layer 202 due to the planarizationtreatment can be inhibited; moreover, variation in the thickness can bereduced.

[Formation of Pair of Electrodes]

Next, a conductive film is formed over the first protective insulatinglayer 211 and the semiconductor layer 202. A resist mask is formed overthe conductive film by a photolithography method or the like andunnecessary portions of the conductive film are etched. Then, the resistmask is removed. Thus, the pair of electrodes 203 can be formed (FIG.6C).

The conductive film can be formed by a sputtering method, an evaporationmethod, a CVD method, or the like, for example.

Here, as illustrated in FIG. 6C, in some cases, an upper portion of thesemiconductor layer 202 is partly etched in the etching of theconductive film to reduce the thickness of a portion where the pair ofelectrodes 203 does not overlap with the semiconductor layer 202. Forthis reason, the semiconductor film serving as the semiconductor layer202 is preferably formed to have a large thickness (i.e., the groove ispreferably formed to have a large depth) in advance in consideration ofthe etching depth.

Although not clearly illustrated in FIG. 6C, in the etching of theconductive film, part of the first protective insulating layer 211 mightbe etched to have a small thickness in some cases.

[Formation of Gate Insulating Layer and Gate Electrode]

Next, an insulating film is formed over the semiconductor layer 202, thepair of electrodes 203, and the first protective insulating layer 211.Next, a conductive film is formed over the insulating film. A resistmask is formed over the conductive film by a photolithography method orthe like and unnecessary portions of the conductive film and theinsulating films are etched. Then, the resist mask is removed. Thus, thegate electrode 205 and the gate insulating layer 204 can be formed (FIG.6D).

The insulating film serving as the gate insulating layer 204 and theconductive film serving as the gate electrode 205 can be formed in amanner similar to those in the example of the manufacturing method 1.

Note that here, the gate insulating layer 204 is etched at the same timewhen the gate electrode 205 is formed, so that the gate insulating layer204 is processed to have a shape similar to that of the gate electrode205 when seen from above. However, the gate insulating layer 204 and thegate electrode 205 may be processed individually so that the gateinsulating layer 204 extends to the outside of the gate electrode 205.At this time, a multi-tone mask such as a gray-tone mask or a half-tonemask is preferably used as a light-exposure mask used in thephotolithography method or the like, in which case the manufacturingprocess can be simplified.

[Formation of Second Protective Insulating Layer]

Next, the second protective insulating layer 212 is formed over thefirst protective insulating layer 211, the pair of electrodes 203, thegate insulating layer 204, and the gate electrode 205 (FIG. 6E).

The second protective insulating layer 212 can be formed by a methodsimilar to that of the first protective insulating layer 211.

Here, the second protective insulating layer 212 is provided in contactwith the first protective insulating layer 211 in a region where thepair of electrodes 203 and the gate electrode 205 are not provided.Thus, the first protective insulating layer 211 and the secondprotective insulating layer 212 can surround the semiconductor layer202.

Through the above steps, the transistor 200 can be manufactured.

[Heat Treatment]

Heat treatment may be performed after the second protective insulatinglayer 212 is formed. Through the heat treatment, oxygen is supplied fromthe first protective insulating layer 211 and the second protectiveinsulating layer 212 to the semiconductor layer 202, whereby oxygenvacancies in the semiconductor layer 202 can be reduced. At this time,release of oxygen from the semiconductor layer 202 is inhibited by thefirst protective insulating layer 211 and the second protectiveinsulating layer 212, so that formation of oxygen vacancies in thesemiconductor layer 202 can be inhibited.

The above is the description of the example of the manufacturing processof the transistor 200.

Modification Examples of Structural Example 2

Structural examples of a transistor that are partly different from thestructure of the transistor described in the structural example 2 aredescribed below. Note that description of the portions already describedis omitted and only different portions are described in detail. Evenwhen positions and shapes of components are different from those in thestructural example 2, the same reference numerals are used as long asthe components have the same functions as those in the structuralexample 2, and detailed description thereof is omitted in some cases.

Modification Example 1

FIGS. 7A and 7B are schematic cross-sectional views of a transistordescribed as an example below. Note that a schematic top view of thetransistor can be referred to FIG. 5A. The transistor illustrated inFIGS. 7A and 7B is different from that in FIGS. 5B and 5C mainly in thatan insulating layer 206 is provided between the semiconductor layer 202and the first protective insulating layer 211.

In a groove provided in the insulating layer 207, the insulating layer206 is provided to cover side surfaces and a top surface of the firstprotective insulating layer 211. The insulating layer 206 is provided tocover the side surfaces and the undersurface of the semiconductor layer202.

The insulating layer 206 provided under the semiconductor layer 202preferably contains an oxide insulating material that releases oxygenwhen heated. With the insulating layer 206 provided under thesemiconductor layer 202, because of heat applied in heat treatment orthe like in the manufacturing process of the transistor, more oxygen canbe supplied to the semiconductor layer 202. Furthermore, with astructure in which the insulating layer 206, the semiconductor layer202, and the like are surrounded by the first protective insulatinglayer 211 and the second protective insulating layer 212, oxygenreleased from the insulating layer 206 is inhibited from being diffusedto the outside (the insulating layer 207 side or above the secondprotective insulating layer 212), which makes it possible to supplyoxygen to the semiconductor layer 202 more effectively.

The insulating layer 206 can be provided to cover also the top surfaceof the first protective insulating layer 211 that does not overlap withthe groove; however, the insulating layer 206 is preferably processed tobe provided inside the groove by the planarization treatment. With sucha structure, the first protective insulating layer 211 and the secondprotective insulating layer 212 are in contact with each other in theregion where the gate electrode 205 and the pair of electrodes 203 arenot provided, so that an oxygen diffusion path is blocked; thus, oxygencan be supplied to the semiconductor layer 202 effectively.

As the insulating layer 206, an oxide insulating film containing oxygenin excess of the stoichiometric composition is preferably used, which issimilar to the case of the above-described insulating layer 106.

Modification Example 2

FIG. 8 illustrates an example of forming a capacitor 220 adjacent to thetransistor 200.

The capacitor 220 includes one of the pair of electrodes 203 of thetransistor 200, an electrode 225 formed by processing the conductivefilm also used for the gate electrode 205, and a dielectric layer 224between the one of the pair of electrodes 203 and the electrode 225 thatis formed by processing the insulating film also used for the gateinsulating layer 204.

Being formed by processing the films used in manufacturing thetransistor 200 in such a manner, the capacitor 220 can be manufacturedat the same time as the transistor 200 without increasing the number ofmanufacturing steps.

Note that although the one of the pair of electrodes 203 of thetransistor 200 is used as one electrode of the capacitor 220 in FIG. 8,one embodiment of the present invention is not limited to this. Anelectrode formed by processing the conductive film also used for thepair of electrodes 203 of the transistor 200 may be used as the oneelectrode of the capacitor 220. Alternatively, at least the gateelectrode 205 and the electrode 225 may be formed as one continuousfilm. Still alternatively, at least the gate insulating layer 204 andthe dielectric layer 224 may be formed as one continuous film

Here, as a material of the insulating film used to form the gateinsulating layer 204 and the dielectric layer 224, a high dielectricconstant material such as aluminum oxide, hafnium oxide, zirconiumoxide, tantalum oxide, titanium oxide, strontium titanate, or bariumtitanate is preferably used. Alternatively, any of these materialscontaining a metal such as lanthanum, aluminum, yttrium, or tungsten ora material containing an oxide of any of these metals may be used. Stillalternatively, a stacked-layer structure of films containing any of theabove materials may be used.

As the insulating film, an oxide insulating film containing oxygen inexcess of the stoichiometric composition is preferably used. With theuse of such an insulating film, due to heat applied in heat treatment orthe like in the manufacturing process of the transistor, oxygen can besupplied from the gate insulating layer 204 to the semiconductor layer202.

Modification Example 3

In the case of arranging a plurality of transistors over a substrate,the integration density of the transistors can be further increased byproviding a groove not for each transistor but for a plurality oftransistors.

As an example, FIGS. 9A to 9D illustrate the case of forming fourtransistors 200, which are connected in series, in an upper portion of agroove provided in the insulating layer 207. FIG. 9A is a schematic topview and FIG. 9B is a schematic cross-sectional view taken along lineI-J in FIG. 9A.

As illustrated in FIG. 9B, the four transistors 200 are formed in theupper portion of the groove formed in the insulating layer 207. The twoadjacent transistors 200 share an electrode 203, and thus are connectedin series. On the other hand, the gate electrode 205 is provided foreach of the transistors 200.

The first protective insulating layer 211 and the second protectiveinsulating layer 212 are provided to surround the four transistors 200and in contact with each other outside the electrodes 203 provided overboth ends of the groove.

FIG. 9C illustrates an example of a circuit configuration which can beused for the transistors 200 connected in series as described above. Acircuit illustrated in FIG. 9C includes four transistors and threecapacitors. In the two adjacent transistors, a source or a drain of oneof the transistors is electrically connected to a source or a drain ofthe other thereof to form a node, and one electrode of the capacitor iselectrically connected to the node.

The structure of the capacitor 220 described in the modification example2 can be used for the capacitor, for example.

The circuit illustrated in FIG. 9C can serve as, for example, a shiftregister by application of a potential described below.

A common potential is applied to the other electrodes of the threecapacitors. In the four transistors, a clock signal (CLK1) is applied togates of the first and third transistors from the left, and a clocksignal (CLK2) is applied to gates of the second and fourth transistorsfrom the left. One of the source and the drain of the first transistoris an input terminal to which an input potential (IN) is applied, andone of the source and the drain of the fourth transistor is an outputterminal from which an output potential (OUT) is output. As CLK1 andCLK2, clock signals having potentials with which adjacent transistorsare alternately turned on and are not in an on state in the same period(e.g., high-level potential) are used, which makes it possible to shiftdata of a potential applied to the input terminal from the left to theright.

FIG. 9D illustrates a configuration in which a plurality of readingtransistors 260 connected in series are added to the circuit illustratedin FIG. 9C. Each of the transistors 260 is electrically connected to anode to which the one electrode of the capacitor is connected. A readingpotential is applied to each of the other electrodes of the capacitors.With such a structure, a circuit illustrated in FIG. 9D can serve as aNAND memory device that can read data of a potential held in the nodeconnected to the one electrode of the capacitor at any time. Here, thetransistor 260 may be a transistor using an oxide semiconductor, whichis similar to the transistor 200, or a transistor using anothersemiconductor, which is described as an example in the followingembodiment.

The above is the description of the modification examples.

This embodiment can be implemented in combination with any of the otherembodiments described in this specification as appropriate.

Embodiment 2

In this embodiment, a structural example of a transistor with astructure partly different from that of the transistor described inEmbodiment 1 as an example is described. Note that description of theportions already described is omitted and only different portions aredescribed in detail. Even when positions and shapes of components aredifferent from those in the above structural examples, the samereference numerals are used as long as the components have the samefunctions as those in the above structural examples, and detaileddescription thereof is omitted in some cases.

A semiconductor device of one embodiment of the present inventionpreferably includes an oxide layer between an oxide semiconductor layerand a gate insulating layer overlapping with the oxide semiconductorlayer, and an oxide layer between the oxide semiconductor layer and aprotective insulating layer overlapping with the oxide semiconductorlayer. The oxide layers each contain at least one metal elementcontained in the oxide semiconductor layer. This makes it possible toinhibit formation of a trap state at the interface between the oxidesemiconductor layer and the insulating layer overlapping with the oxidesemiconductor layer; accordingly, deterioration of the electricalcharacteristics of the transistor can be suppressed.

That is, one embodiment of the present invention preferably has astructure in which at least a top surface and an undersurface of anoxide semiconductor layer are covered with an oxide layer serving as abarrier film that prevents formation of an interface state at aninterface with the oxide semiconductor layer, the top surface and sidesurfaces of the oxide semiconductor layer in the channel width directionare covered with a gate electrode with a gate insulating layer providedtherebetween, and insulating layers each including an aluminum oxidefilm are provided to wrap the oxide semiconductor layer. This structuremakes it possible to inhibit generation of oxygen vacancies and mixingof impurities, which cause carriers in the oxide semiconductor layer andan interface with the oxide semiconductor layer, so that the oxidesemiconductor layer is highly purified to be an intrinsic oxidesemiconductor layer. The expression being “highly purified to be anintrinsic oxide semiconductor layer” refers to being purified orsubstantially purified to be an intrinsic or substantially intrinsicoxide semiconductor layer. Consequently, a change in the electricalcharacteristics of a transistor including the oxide semiconductor layeris suppressed, which enables a semiconductor device to have highreliability.

Note that in this specification and the like, in the case of thesubstantially purified oxide semiconductor layer, the carrier densitythereof is lower than 1×10¹⁷/cm³, lower than 1×10¹⁵/cm³, or lower than1×10¹³/cm³. With a highly purified intrinsic oxide semiconductor layer,the transistor can have stable electric characteristics.

More specifically, the following structures can be employed for example.

Structural Example 1

FIGS. 10A and 10B are schematic cross-sectional views of a transistor150 described as an example below. Note that a schematic top view of thetransistor 150 can be referred to FIG. 1A. The transistor 150illustrated in FIGS. 10A and 10B differs from the transistor 100described as an example in Embodiment 1 mainly in that a first oxidelayer 151 and a second oxide layer 152 are included.

The first oxide layer 151 is provided between the first protectiveinsulating layer 111 and the semiconductor layer 102. The second oxidelayer 152 is provided between the semiconductor layer 102 and the gateinsulating layer 104.

Specifically, a top surface of the second oxide layer 152 is in contactwith undersurfaces of the pair of electrodes 103 and an undersurface ofthe gate insulating layer 104.

The first oxide layer 151 and the second oxide layer 152 each contain anoxide containing one or more of the metal elements contained in thesemiconductor layer 102.

Note that the boundary between the semiconductor layer 102 and the firstoxide layer 151 or the boundary between the semiconductor layer 102 andthe second oxide layer 152 is not clear in some cases.

For example, the first oxide layer 151 and the second oxide layer 152each contain In or Ga; the first oxide layer 151 and the second oxidelayer 152 each contain, for example, a material typified by anIn—Ga-based oxide, an In—Zn-based oxide, or an In-M-Zn-based oxide (M isAl, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). In addition, the energy of theconduction band minimum of the material is closer to a vacuum level thanthat of the semiconductor layer 102 is, and typically, the differencebetween the energy of the conduction band minimum of the first oxidelayer 151 or the second oxide layer 152 and the energy of the conductionband minimum of the semiconductor layer 102 is preferably 0.05 eV orhigher, 0.07 eV or higher, 0.1 eV or higher, or 0.15 eV or higher and 2eV or lower, 1 eV or lower, 0.5 eV or lower, or 0.4 eV or lower.

An oxide having a Ga (serving as a stabilizer) content higher than thatof the semiconductor layer 102 is used for the first oxide layer 151 andthe second oxide layer 152, between which the semiconductor layer 102 issandwiched, in which case release of oxygen from the semiconductor layer102 can be inhibited.

When an In—Ga—Zn-based oxide in which the atomic ratio of In to Ga andZn is 1:1:1 or 3:1:2 is used for the semiconductor layer 102, forexample, an In—Ga—Zn-based oxide in which the atomic ratio of In to Gaand Zn is 1:3:2, 1:3:4, 1:3:6, 1:6:4, 1:6:8, 1:6:10, or 1:9:6 can beused for the first oxide layer 151 or the second oxide layer 152. Notethat the atomic ratio of each of the semiconductor layer 102, the firstoxide layer 151, and the second oxide layer 152 may vary within a rangeof ±20% of any of the above-described atomic ratios as an error. For thefirst oxide layer 151 and the second oxide layer 152, materials with thesame composition or material with different compositions may be used.

Further, when an In-M-Zn-based oxide is used for the semiconductor layer102, an oxide containing metal elements in the atomic ratio satisfyingthe following conditions is preferably used for a target for forming thesemiconductor film serving as the semiconductor layer 102. Given thatthe atomic ratio of the metal elements in the oxide is In:M:Zn=x₁:y₁:z₁,x₁/y₁ is greater than or equal to ⅓ and less than or equal to 6,preferably greater than or equal to 1 and less than or equal to 6, andz₁/y₁ is greater than or equal to ⅓ and less than or equal to 6,preferably greater than or equal to 1 and less than or equal to 6. Notethat when z₁/y₁ is less than or equal to 6, a CAAC-OS film to bedescribed later is easily formed. Typical examples of the atomic ratioof the metal elements in the target are In:M:Zn=1:1:1, In:M:Zn=3:1:2, orthe like.

When an In-M-Zn-based oxide is used for the first oxide layer 151 andthe second oxide layer 152, an oxide containing metal elements in thefollowing atomic ratio is preferably used for a target for forming oxidefilms serving as the first oxide layer 151 and the second oxide layer152. Given that the atomic ratio of the metal elements in the oxide isIn:M:Zn=x₂:y₂:z₂, it is preferable that x₂/y₂ be less than x₁/y₁, andz₂/y₂ be greater than or equal to ⅓ and less than or equal to 6,preferably greater than or equal to 1 and less than or equal to 6. Notethat when z₂/y₂ is less than or equal to 6, a CAAC-OS film to bedescribed later is easily formed. Typical examples of the atomic ratioof the metal elements in the target are In:M:Zn=1:3:4, In:M:Zn 1:3:6,In:M:Zn=1:3:8, or the like.

By using a material in which the energy level of the conduction bandminimum is closer to the vacuum level than that of the semiconductorlayer 102 is for the first oxide layer 151 and the second oxide layer152, a channel is mainly formed in the semiconductor layer 102, so thatthe semiconductor layer 102 serves as a main current path. When thesemiconductor layer 102 in which a channel is formed is sandwichedbetween the first oxide layer 151 and the second oxide layer 152 asdescribed above, generation of interface states between these layers issuppressed, and thus reliability of the electrical characteristics ofthe transistor is improved.

Note that, without limitation to those described above, a material withan appropriate composition may be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of a transistor. In orderto obtain the required semiconductor characteristics of the transistor,it is preferable that the carrier density, the impurity concentration,the defect density, the atomic ratio of a metal element to oxygen, theinteratomic distance, the density, and the like of each of thesemiconductor layer 102, the first oxide layer 151, and the second oxidelayer 152 be set to appropriate values.

Here, the thickness of the semiconductor layer 102 is preferably largerthan that of the first oxide layer 151. The thicker the semiconductorlayer 102 is, the larger the on-state current of the transistor is. Thethickness of the first oxide layer 151 may be set as appropriate as longas formation of an interface state at an interface with thesemiconductor layer 102 is inhibited. For example, the thickness of thesemiconductor layer 102 is larger than that of the first oxide layer151, preferably 2 times or more, further preferably 4 times or more,still further preferably 6 times or more as large as that of the firstoxide layer 151.

The thickness of the second oxide layer 152 may be set as appropriate,in a manner similar to that of the first oxide layer 151, as long asformation of an interface state at an interface with the semiconductorlayer 102 is inhibited. For example, the thickness of the second oxidelayer 152 may be set smaller than or equal to that of the first oxidelayer 151. The second oxide layer 152 preferably has a small thicknessbecause the thick second oxide layer 152 might make it difficult for anelectric field by the gate electrode 105 to extend to the semiconductorlayer 102. Note that the thickness of the second oxide layer 152 is notlimited to the above, and may be set as appropriate depending on adriving voltage of the transistor 150 in consideration of thewithstanding voltage of the gate insulating layer 104.

In addition, as illustrated in FIG. 10B, it is preferable that the topsurface of the first protective insulating layer 111 in the vicinity ofthe oxide layer 151 be positioned lower than that of an undersurface ofthe oxide layer 151, and the lower parts of the side surfaces of thesemiconductor layer 102 be surrounded by the gate electrode 105.Consequently, the electric field by the gate electrode 105 issufficiently applied to the lower parts of the side surface of thesemiconductor layer 102, which makes it possible to increase theon-state current of the transistor 150. As illustrated in FIGS. 27A and27B, the undersurface of the gate electrode 105 is preferably positionedlower than the undersurface of the oxide layer 151, in which case theon-state current of the transistor 150 can be further increased in amanner similar to the above.

Here, a band structure of a channel formation region in the transistor150 is described.

FIGS. 11A and 11B each schematically illustrate a band structure of achannel formation region in the thickness direction.

In FIGS. 11A and 11B, EcI1, EcS1, EcS2, EcS3, EcI2 schematicallyrepresent energies at the conduction band minimum of the firstprotective insulating layer 111, the first oxide layer 151, thesemiconductor layer 102, the second oxide layer 152, and the gateinsulating layer 104, respectively. Note that the thicknesses of thelayers are not considered here for convenience.

Here, an energy difference between the vacuum level and the conductionband minimum (the difference is also referred to as electron affinity)corresponds to a value obtained by subtracting an energy gap from anenergy difference between the vacuum level and the valence band maximum(the difference is also referred to as an ionization potential). Notethat the energy gap can be measured using a spectroscopic ellipsometer(e.g., UT-300 manufactured by HORIBA JOBIN YVON S.A.S.). Note that theenergy difference between the vacuum level and the valence band maximumcan be measured using an ultraviolet photoelectron spectroscopy (UPS)device (e.g., VersaProbe manufactured by ULVAC-PHI, Inc.).

As shown in FIG. 11A, energy at the conduction band minimum continuouslychanges from the first oxide layer 151 through the semiconductor layer102 to the second oxide layer 152, without an energy barriertherebetween. This is because oxygen is easily diffused between thefirst oxide layer 151 and the semiconductor layer 102, and between thesemiconductor layer 102 and the second oxide layer 152 since theselayers have similar compositions, and thus a layer what is called amixed layer is formed therebetween.

Note that although the case where the first oxide layer 151 and thesecond oxide layer 152 are oxide layers having the same energy gap isshown in FIG. 11A, the first oxide layer and the second oxide layer maybe oxide layers having different energy gaps. For example, FIG. 11Bshows part of the band structure in which EcS3 is higher than EcS1.Although not shown, EcS1 may be higher than EcS3.

FIGS. 11A and 11B show that in the channel formation region, thesemiconductor layer 102 serves as a well and a channel is formed in thesemiconductor layer 102. Note that since the energies at the conductionband minimum are changed continuously, the first oxide layer 151, thesemiconductor layer 102, and the second oxide layer 152 can also bereferred to as U-shaped wells. Further, a channel formed to have such astructure can also be referred to as a buried channel.

The first oxide layer 151 and the second oxide layer 152 are oxides thatcontain one or more of the metal elements contained in the semiconductorlayer 102; therefore, a stacked-layer structure in which the first oxidelayer 151, the semiconductor layer 102, and the second oxide layer 152are stacked can also be referred to as an oxide stack including layershaving the same main component. Hereinafter, the stacked-layer structurein which the first oxide layer 151, the semiconductor layer 102, and thesecond oxide layer 152 are stacked is also referred to as an oxidestack. The oxide stack including the layers having the same maincomponent is formed not simply by stacking layers but to have acontinuous junction (here, in particular, a U-shaped well structure inwhich energy of the conduction band minimum is changed continuouslybetween the layers). This is because when impurities which form a defectstate such as a trap center or a recombination center are mixed at aninterface between the layers, the continuity of the energy band is lost,and thus carriers are trapped or disappear by recombination at theinterface.

To form a continuous junction, the layers are preferably stackedsuccessively without exposure to the air with use of a multi-chamberdeposition apparatus (e.g., a sputtering apparatus) including a loadlock chamber. Each chamber in the sputtering apparatus is preferablyevacuated to high vacuum (approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with anentrapment vacuum evacuation pump such as a cryopump so that water orthe like, which is an impurity for an oxide semiconductor, is removed asmuch as possible. Alternatively, a turbo molecular pump and a cold trapare preferably used in combination to prevent backflow of gas into thechamber through an evacuation system.

Not only high vacuum evacuation in a chamber but also high purity of asputtering gas is necessary to obtain a high-purity intrinsic oxidesemiconductor. When a highly purified gas having a dew point of −40° C.or lower, preferably −80° C. or lower, more preferably −100° C. or loweris used as an oxygen gas or an argon gas used as a sputtering gas,moisture or the like can be prevented from entering an oxidesemiconductor as much as possible.

The first oxide layer 151 under the semiconductor layer 102 and thesecond oxide layer 152 over the semiconductor layer 102 can function asbarrier layers to inhibit the influence of a trap state, which is formedat the interface between the oxide stack and the insulating layer (thefirst protective insulating layer 111 and the gate insulating layer 104)in contact with the oxide stack, on the semiconductor layer 102 thatserves as the main carrier path in the transistor.

For example, oxygen vacancies contained in the semiconductor layerappear as localized states in deep energy area in the energy gap of theoxide semiconductor. A carrier is trapped in such localized states, sothat reliability of the transistor is lowered. For this reason, oxygenvacancies contained in the semiconductor layer should be reduced. In theoxide stack, the oxide layers in which oxygen vacancy is less likely tobe generated than in the semiconductor layer 102 are provided on andunder the semiconductor layer 102 to be in contact with thesemiconductor layer 102, whereby oxygen vacancy in the semiconductorlayer 102 can be reduced. For example, in the semiconductor layer 102,the absorption coefficient due to the localized levels, which isobtained by measurement by a constant photocurrent method (CPM) is setlower than 1×10⁻³/cm, preferably lower than 1×10⁻⁴/cm.

In addition, when the semiconductor layer 102 is in contact with aninsulating layer including a different constituent element (e.g., a baseinsulating layer including a silicon oxide film), an interface state issometimes formed at the interface of the two layers and the interfacestate forms a channel. At this time, a second transistor having adifferent threshold voltage appears, so that an apparent thresholdvoltage of the transistor is varied. However, since the first oxidelayer 151 contains one or more metal elements forming the semiconductorlayer 102 in the oxide stack, an interface state is less likely to beformed at the interface between the first oxide layer 151 and thesemiconductor layer 102. Thus, the formation of the first oxide layer151 makes it possible to reduce fluctuation in the electricalcharacteristics of the transistor, such as threshold voltage.

When a channel is formed at the interface between the gate insulatinglayer 104 and the semiconductor layer 102, interface scattering occursat the interface and the field-effect mobility of the transistor isreduced. However, since the second oxide layer 152 contains one or moremetal elements forming the semiconductor layer 102 in the oxide stack,scattering of carriers is less likely to occur at the interface betweenthe second oxide layer 152 and the semiconductor layer 102, and thus thefield-effect mobility of the transistor can be increased.

Modification Example 2

FIGS. 12A and 12B are schematic cross-sectional views of a transistor160 described as an example below. Note that a schematic top view of thetransistor 160 can be referred to FIG. 1A. A main difference between thetransistor 160 illustrated in FIGS. 12A and 12B and the above-describedtransistor 150 is the shape of the second oxide layer 152.

In the transistor 160, an undersurface of the second oxide layer 152 isin contact with top surfaces of the pair of electrodes 103. Furthermore,the second oxide layer 152 is in contact with the top surface and theside surfaces of the semiconductor layer 102 in a region where the pairof electrodes 103 is not provided.

In the structure illustrated in FIGS. 12A and 12B, the second oxidelayer 152, the gate insulating layer 104, and the gate electrode 105 areprocessed with the use of one photomask so that the shapes of the secondoxide layer 152, the gate insulating layer 104, and the gate electrode105 are aligned with one another when seen from above. The secondprotective insulating layer 112 is in contact with end portions of thesecond oxide layer 152 and the gate insulating layer 104. Such astructure can inhibit release of oxygen from the semiconductor layer 102through the end portions of the second oxide layer 152 and the gateinsulating layer 104.

As illustrated in FIG. 12B, in the transistor 160, not only the topsurface of the semiconductor layer 102 but also the side surfacesthereof are in contact with the second oxide layer 152. That is, thechannel formation region in the semiconductor layer 102 is surrounded bythe first oxide layer 151 and the second oxide layer 152.

With such a structure, the second oxide layer 152 in contact with theside surfaces of the semiconductor layer 102 can inhibit formation of aninterface state at an interface with the semiconductor layer 102 even onthe side surfaces of the semiconductor layer 102. Consequently, even inthe case of actively using a channel formed in the vicinity of the sidesurfaces of the semiconductor layer 102, a change in the electricalcharacteristics of the transistor can be suppressed, which makes itpossible to provide a transistor with high on-state current and highreliability.

In addition, as illustrated in FIG. 12B, it is preferable that the topsurface of the first protective insulating layer 111 in the vicinity ofthe oxide layer 151 be positioned lower than that of the undersurface ofthe oxide layer 151, and the lower parts of the side surfaces of thesemiconductor layer 102 be surrounded by the gate electrode 105.Consequently, the electric field by the gate electrode 105 issufficiently applied to the lower parts of the side surface of thesemiconductor layer 102, which makes it possible to increase theon-state current of the transistor 160. As illustrated in FIGS. 28A and28B, the undersurface of the gate electrode 105 is preferably positionedlower than the undersurface of the oxide layer 151, in which case theon-state current of the transistor 160 can be further increased in amanner similar to the above.

In addition, the insulating layer 106 that releases oxygen when heated,which is described as an example in Embodiment 1, can also be used.

FIGS. 13A and 13B are schematic cross-sectional views of a transistor170 with a structure different from that of the transistor 160.

The transistor 170 differs from the transistor 160 mainly in that theinsulating layer 106 is included between the first oxide layer 151 andthe first protective insulating layer 111.

As illustrated in FIGS. 13A and 13B, the semiconductor layer 102, thefirst oxide layer 151, and the insulating layer 106 are processed tohave island shapes, the second oxide layer 152 is provided to cover thesemiconductor layer 102, the first oxide layer 151, and the insulatinglayer 106, and the first protective insulating layer 111 is providedbelow the second oxide layer 152, the semiconductor layer 102, the firstoxide layer 151, and the insulating layer 106; thus, oxygen releasedfrom the insulating layer 106 can be supplied to the semiconductor layer102 through the first oxide layer 151 more effectively.

Furthermore, as illustrated in FIG. 13B, it is preferable that the topsurface of the first protective insulating layer 111 in the vicinity ofthe insulating layer 106 be positioned lower than that of anundersurface of the insulating layer 106, and the lower parts of theside surfaces of the semiconductor layer 102 be surrounded by the gateelectrode 105. Consequently, the electric field by the gate electrode105 is sufficiently applied to the lower parts of the side surface ofthe semiconductor layer 102, which makes it possible to increase theon-state current of the transistor 170. As illustrated in FIGS. 29A and29B, the undersurface of the gate electrode 105 is preferably positionedlower than the undersurface of the oxide layer 151, in which case theon-state current of the transistor 170 can be further increased in amanner similar to the above.

FIGS. 13C and 13D are schematic cross-sectional view of a transistor 180with a structure partly different from that of the transistor 170. Thetransistor 180 includes the insulating layer 106, the first oxide layer151, and the second oxide layer 152 which are not processed to haveisland shapes. Such a structure can be formed by using a material with awide band gap for the first oxide layer 151 and the second oxide layer152.

Note that in the case of the transistor 180, it is preferable that, in aregion that is not illustrated, the insulating layer 106, the firstoxide layer 151, and the second oxide layer 152 be etched to provide aregion where the first protective insulating layer 111 and the secondprotective insulating layer 112 are in contact with each other. Forexample, a plurality of transistors may be formed in a region surroundedby the first protective insulating layer 111 and the second protectiveinsulating layer 112.

Furthermore, as illustrated in FIG. 13D, it is preferable that the topsurface of the first protective insulating layer 111 in the vicinity ofthe semiconductor layer 102 be positioned lower than that of theundersurface of the semiconductor layer 102, and the lower parts of theside surfaces of the semiconductor layer 102 be surrounded by the gateelectrode 105. Consequently, the electric field by the gate electrode105 is sufficiently applied to the lower parts of the side surface ofthe semiconductor layer 102, which makes it possible to increase theon-state current of the transistor 180. As illustrated in FIGS. 29C and29D, the undersurface of the gate electrode 105 is preferably positionedlower than the undersurface of the semiconductor layer 102, in whichcase the on-state current of the transistor 180 can be further increasedin a manner similar to the above.

Structural Example 3

FIGS. 14A and 14B are schematic cross-sectional views of a transistor250 described as an example below. Note that a schematic top view of thetransistor 250 can be referred to FIG. 1A. The transistor 250illustrated in FIGS. 14A and 14B differs from the transistor 200described as an example in Embodiment 1 mainly in that a first oxidelayer 251 and a second oxide layer 252 are included.

The first oxide layer 251 is provided between the first protectiveinsulating layer 211 and the semiconductor layer 202. The second oxidelayer 252 is provided between the semiconductor layer 202 and the gateinsulating layer 204.

More specifically, in a groove provided in the insulating layer 207, thefirst oxide layer 251 is provided to cover the side surfaces and the topsurface of the first protective insulating layer 211. The first oxidelayer 251 is provided to be in contact with the side surfaces and theundersurface of the semiconductor layer 202.

An undersurface of the second oxide layer 252 is in contact with topsurfaces of the pair of electrodes 203. Furthermore, the second oxidelayer 252 is in contact with the top surface of the semiconductor layer202 in a region where the pair of electrodes 203 is not provided.

The first oxide layer 251 and the second oxide layer 252 each contain anoxide containing one or more of the metal elements contained in thesemiconductor layer 202.

Note that the boundary between the semiconductor layer 202 and the firstoxide layer 251 or the boundary between the semiconductor layer 202 andthe second oxide layer 252 is not clear in some cases.

For the first oxide layer 251 and the second oxide layer 252, materialssimilar to those of the first oxide layer 151 and the second oxide layer152 can be used, for example.

In the structure illustrated in FIGS. 14A and 14B, the second oxidelayer 252, the gate insulating layer 204, and the gate electrode 205 areprocessed with the use of one photomask so that the shapes of the secondoxide layer 252, the gate insulating layer 204, and the gate electrode205 are aligned with one another when seen from above. The secondprotective insulating layer 212 is in contact with end portions of thesecond oxide layer 252 and the gate insulating layer 204. Such astructure can inhibit release of oxygen from the semiconductor layer 202through the end portions of the second oxide layer 252 and the gateinsulating layer 204.

As illustrated in FIG. 14B, in the transistor 250, not only theundersurface of the semiconductor layer 202 but also the side surfacesthereof are in contact with the first oxide layer 251, and the topsurface of the semiconductor layer 202 is in contact with the secondoxide layer 252. That is, the channel formation region in thesemiconductor layer 202 is surrounded by the first oxide layer 251 andthe second oxide layer 252.

Such a structure can inhibit formation of an interface state at aninterface with a surface of the channel formation region in thesemiconductor layer 202. Thus, a change in the electricalcharacteristics of the transistor can be suppressed, which makes itpossible to provide a highly reliable transistor.

Here, the thickness of the semiconductor layer 202 is preferably atleast larger than that of the first oxide layer 251. The thicker thesemiconductor layer 202 is, the larger the on-state current of thetransistor is. The thickness of the first oxide layer 251 may be set asappropriate as long as formation of an interface state at an interfacewith the semiconductor layer 202 is inhibited. For example, thethickness of the semiconductor layer 202 is larger than that of thefirst oxide layer 251, preferably 2 times or more, further preferably 4times or more, still further preferably 6 times or more as large as thatof the first oxide layer 251.

The depth of the groove provided in the insulating layer 207 may be setas appropriate in consideration of the thicknesses of the firstprotective insulating layer 211, the first oxide layer 251, and thesemiconductor layer 202 which have been processed. The width of thegroove may be set as appropriate depending on the channel length and thechannel width of the transistor 250.

The thickness of the second oxide layer 252 may be set as appropriate,in a manner similar to that of the first oxide layer 251, as long asformation of an interface state at an interface with the semiconductorlayer 202 is inhibited. For example, the thickness of the second oxidelayer 252 may be set smaller than or equal to that of the first oxidelayer 251. The second oxide layer 252 preferably has a small thicknessbecause the thick second oxide layer 252 might make it difficult for anelectric field by the gate electrode 205 to extend to the semiconductorlayer 202. Note that the thickness of the second oxide layer 252 is notlimited to the above, and may be set as appropriate depending on adriving voltage of the transistor 250 in consideration of thewithstanding voltage of the gate insulating layer 204.

In addition, the insulating layer 206 that releases oxygen when heated,which is described as an example in the modification example 1 inEmbodiment 1, can also be used.

FIGS. 14C and 14D are schematic cross-sectional views of a transistor270 with a structure different from that of the transistor 250.

The transistor 270 differs from the transistor 250 mainly in that theinsulating layer 206 is provided between the first oxide layer 251 andthe first protective insulating layer 211, and the semiconductor layer202 covers a groove.

With the first oxide layer 251 provided to fill the groove, the physicaldistance between the semiconductor layer 202 and the insulating layer206 can be large in the channel formation region. Thus, the number ofinterface states formed at an interface with the semiconductor layer 202can be further reduced in the channel formation region.

FIG. 15 illustrates the case of connecting a plurality of transistors280 with different structure from those of the transistors 250 and 270in series. The transistor 280 differs from the transistor 270 mainly inthat the first oxide layer 251 is provided to cover a groove.

The insulating layer 206 is embedded in the groove and the first oxidelayer 251 and the semiconductor layer 202 are provided thereover; thus,the volume of the insulating layer 206 can be easily increased.Consequently, the amount of oxygen supplied to the semiconductor layer202 can be increased. Furthermore, with such a structure, steps are notformed on a top surface of the insulating layer 206; therefore, theinsulating layer 206 can have a large thickness without decreasing thecoverage with the first oxide layer 251, the semiconductor layer 202,and the like which are formed over the insulating layer 206.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 3

An oxide semiconductor that can be favorably used in a semiconductordevice of one embodiment of the present invention is described in thisembodiment.

An oxide semiconductor has a wide energy gap of 3.0 eV or higher. Atransistor using an oxide semiconductor film obtained by processing ofthe oxide semiconductor in an appropriate condition and a sufficientreduction in carrier density of the oxide semiconductor can have muchlower leakage current between a source and a drain in an off state(off-state current) than a conventional transistor using silicon.

An applicable oxide semiconductor preferably contains at least indium(In) or zinc (Zn). In particular, In and Zn are preferably contained. Inaddition, as a stabilizer for reducing variation in electriccharacteristics of the transistor using the oxide semiconductor, one ormore selected from gallium (Ga), tin (Sn), hafnium (Hf), zirconium (Zr),titanium (Ti), scandium (Sc), yttrium (Y), and an lanthanoid (such ascerium (Ce), neodymium (Nd), or gadolinium (Gd)) is preferablycontained.

As the oxide semiconductor, for example, any of the following can beused: indium oxide, tin oxide, zinc oxide, an In—Zn-based oxide, aSn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, an In—Ga-based oxide, anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide,an In—Zr—Zn-based oxide, an In—Ti—Zn-based oxide, an In—Sc—Zn-basedoxide, an In—Y—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-basedoxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or anIn—Hf—Al—Zn-based oxide.

Here, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Znas its main components and there is no particular limitation on theratio of In to Ga and Zn. The In—Ga—Zn-based oxide may contain anothermetal element in addition to In, Ga, and Zn.

Alternatively, as the oxide semiconductor, a material represented byInMO₃(ZnO)_(m) (m>0 is satisfied, and m is not an integer) may be used.Note that M represents one or more metal elements selected from Ga, Fe,Mn, and Co, or the above-described element as a stabilizer. Stillalternatively, as the oxide semiconductor, a material represented by achemical formula, In₂SnO₅(ZnO)_(n) (n>0 is satisfied, and n is aninteger) may be used.

For example, an In—Ga—Zn-based oxide in which an atomic ratio of In toGa and Zn is 1:1:1, 1:3:2, 1:3:4, 1:3:6, 3:1:2, or 2:1:3, or an oxidewhose composition is in the neighborhood of the above compositions maybe used.

When an oxide semiconductor film contains a large amount of hydrogen,the hydrogen and an oxide semiconductor are bonded to each other, sothat part of the hydrogen serves as a donor and causes generation of anelectron which is a carrier. As a result, the threshold voltage of thetransistor shifts in the negative direction. For this reason, it ispreferable that, after formation of the oxide semiconductor film,dehydration treatment (dehydrogenation treatment) be performed to removehydrogen or moisture from the oxide semiconductor film so that the oxidesemiconductor film is highly purified to contain impurities as little aspossible.

Note that oxygen in the oxide semiconductor film is also reduced by thedehydration treatment (dehydrogenation treatment) in some cases.Therefore, it is preferable that oxygen be added to the oxidesemiconductor film to fill oxygen vacancies increased by the dehydrationtreatment (dehydrogenation treatment). In this specification and thelike, supplying oxygen to an oxide semiconductor film may be expressedas oxygen adding treatment or treatment for making the oxygen content ofan oxide semiconductor film be in excess of that in the stoichiometriccomposition may be expressed as treatment for making an oxygen-excessstate.

In this manner, hydrogen or moisture is removed from the oxidesemiconductor film by the dehydration treatment (dehydrogenationtreatment) and oxygen vacancies therein are filled by the oxygen addingtreatment, whereby the oxide semiconductor film can be turned into ani-type (intrinsic) oxide semiconductor film or a substantially i-type(intrinsic) oxide semiconductor film which is extremely close to ani-type oxide semiconductor film. Note that “substantially intrinsic”means that the oxide semiconductor film contains extremely few (close tozero) carriers derived from a donor and has a carrier density of1×10¹⁷/cm³ or lower, 1×10¹⁶/cm³ or lower, 1×10¹⁵/cm³ or lower,1×10¹⁴/cm³ or lower, 1×10¹³/cm³ or lower.

In this manner, a transistor including an i-type or substantially i-typeoxide semiconductor film can have extremely favorable off-state currentcharacteristics. For example, the drain current at the time when thetransistor including the oxide semiconductor film is in an off-state canbe less than or equal to 1×10⁻¹⁸ A, preferably less than or equal to1×10⁻²¹ A, further preferably less than or equal to 1×10⁻²⁴ A at roomtemperature (approximately 25° C.); or less than or equal to 1×10⁻¹⁵ A,preferably less than or equal to 1×10⁻¹⁸ A, further preferably less thanor equal to 1×10⁻²¹ A at 85° C. Note that an off state of an n-channeltransistor refers to a state where the gate voltage is sufficientlylower than the threshold voltage. Specifically, the transistor is in anoff state when the gate voltage is lower than the threshold voltage by 1V or more, 2 V or more, or 3 V or more.

A structure of an oxide semiconductor film is described below.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.

First, the CAAC-OS film is described.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Inaddition, a term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

The CAAC-OS film is an oxide semiconductor film having a plurality ofc-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

FIG. 23A is a cross-sectional TEM image of a CAAC-OS film. FIG. 23B is across-sectional TEM image obtained by enlarging the image of FIG. 23A.In FIG. 23B, atomic arrangement is highlighted for easy understanding.

FIG. 23C is Fourier transform images of regions each surrounded by acircle (the diameter is approximately 4 nm) between A and O and betweenO and A′ in FIG. 23A. C-axis alignment can be observed in each region inFIG. 23C. The c-axis direction between A and O is different from thatbetween O and A′, which indicates that a grain in the region between Aand O is different from that between O and A′. In addition, between Aand O, the angle of the c-axis continuously and gradually changes from14.3°, 16.6° to 26.4°. Similarly, between O and A′, the angle of thec-axis continuously changes from −18.3°, −17.6°, to −15.9°.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(bright spots) having alignment are shown. For example, when electrondiffraction with an electron beam having a diameter of 1 nm or greaterand 30 nm or less (such electron diffraction is also referred to asnanobeam electron diffraction) is performed on the top surface of theCAAC-OS film, spots are observed (see FIG. 24A).

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in the planTEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while′ the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface of the CAAC-OS film. Thus, for example, in thecase where a shape of the CAAC-OS film is changed by etching or thelike, the c-axis might not be necessarily parallel to a normal vector ofa formation surface or a normal vector of a top surface of the CAAC-OSfilm.

Furthermore, distribution of c-axis aligned crystal parts in the CAAC-OSfilm is not necessarily uniform. For example, in the case where crystalgrowth leading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases. Further, inthe CAAC-OS film to which an impurity is added, a region to which theimpurity is added is altered, and the proportion of the c-axis alignedcrystal parts in the CAAC-OS film varies depending on regions, in somecases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has a higherstrength of binding to oxygen than that of a metal element included inthe oxide semiconductor film, such as silicon, disturbs the atomicarrangement of the oxide semiconductor film by depriving the oxidesemiconductor film of oxygen and causes a decrease in crystallinity.Further, a heavy metal such as iron or nickel, argon, carbon dioxide, orthe like has a large atomic radius (molecular radius), and thus disturbsthe atomic arrangement of the oxide semiconductor film and causes adecrease in crystallinity when it is contained in the oxidesemiconductor film. Note that the impurity contained in the oxidesemiconductor film might serve as a carrier trap or a carrier generationsource.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Charge trapped by the carrier traps in the oxidesemiconductor film takes a long time to be released, and might behavelike fixed charge. Thus, the transistor which includes the oxidesemiconductor film having high impurity concentration and a high densityof defect states has unstable electrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, the microcrystalline oxide semiconductor film is described.

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor film in some cases. In mostcases, a crystal part in the microcrystalline oxide semiconductor filmis greater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In an image obtained withTEM, a grain boundary cannot be found clearly in the nc-OS film in somecases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. There is noregularity of crystal orientation between different crystal parts in thenc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor depending on an analysis method. Forexample, when the nc-OS film is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Further, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film which isobtained by using an electron beam having a probe diameter (e.g.,greater than or equal to 50 nm) larger than the diameter of a crystalpart. Meanwhile, spots are shown in a nanobeam electron diffractionpattern of the nc-OS film obtained by using an electron beam having aprobe diameter close to, or smaller than the diameter of a crystal part.Further, in a nanobeam electron diffraction pattern of the nc-OS film,regions with high luminance in a circular (ring) pattern are shown insome cases. Also in a nanobeam electron diffraction pattern of the nc-OSfilm, a plurality of spots is shown in a ring-like region in some cases.

Since the nc-OS film is an oxide semiconductor film having moreregularity than the amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

In the case where the oxide semiconductor film has a plurality ofstructures, the structures can be analyzed using nanobeam electrondiffraction in some cases.

FIG. 24C illustrates a transmission electron diffraction measurementapparatus which includes an electron gun chamber 10, an optical system12 below the electron gun chamber 10, a sample chamber 14 below theoptical system 12, an optical system 16 below the sample chamber 14, anobservation chamber 20 below the optical system 16, a camera 18installed in the observation chamber 20, and a film chamber 22 below theobservation chamber 20. The camera 18 is provided to face toward theinside of the observation chamber 20. Note that the film chamber 22 isnot necessarily provided.

FIG. 24D illustrates an internal structure of the transmission electrondiffraction measurement apparatus illustrated in FIG. 24C. In thetransmission electron diffraction measurement apparatus, a substance 28which is positioned in the sample chamber 14 is irradiated withelectrons emitted from an electron gun installed in the electron gunchamber 10 through the optical system 12. Electrons passing through thesubstance 28 enter a fluorescent plate 32 provided in the observationchamber 20 through the optical system 16. A pattern which depends on theintensity of the incident electrons appears in the fluorescent plate 32,so that the transmitted electron diffraction pattern can be measured.

The camera 18 is installed so as to face the fluorescent plate 32 andcan take a picture of a pattern appearing in the fluorescent plate 32.An angle formed by a straight line which passes through the center of alens of the camera 18 and the center of the fluorescent plate 32 and anupper surface of the fluorescent plate 32 is, for example, 15° or moreand 80° or less, 30° or more and 75° or less, or 45° or more and 70° orless. As the angle is reduced, distortion of the transmission electrondiffraction pattern taken by the camera 18 becomes larger. Note that ifthe angle is obtained in advance, the distortion of an obtainedtransmission electron diffraction pattern can be corrected. Note thatthe film chamber 22 may be provided with the camera 18. For example, thecamera 18 may be set in the film chamber 22 so as to be opposite to theincident direction of electrons 24. In this case, a transmissionelectron diffraction pattern with less distortion can be taken from therear surface of the fluorescent plate 32.

A holder for fixing the substance 28 that is a sample is provided in thesample chamber 14. The holder transmits electrons passing through thesubstance 28. The holder may have, for example, a function of moving thesubstance 28 in the direction of the X, Y, and Z axes. The movementfunction of the holder may have an accuracy of moving the substancewithin the range of, for example, 1 nm to 10 nm, 5 nm to 50 nm, 10 nm to100 nm, 50 nm to 500 nm, and 100 nm to 1 μm. The range is preferablydetermined to be an optimal range for the structure of the substance 28.

Then, a method for measuring a transmission electron diffraction patternof a substance by the transmission electron diffraction measurementapparatus described above is described.

For example, changes in the structure of a substance can be observed bychanging (scanning) the irradiation position of the electrons 24 thatare a nanobeam in the substance, as illustrated in FIG. 24D. At thistime, when the substance 28 is a CAAC-OS film, a diffraction patternshown in FIG. 24A can be observed. When the substance 28 is an nc-OSfilm, a diffraction pattern shown in FIG. 24B can be observed.

Even when the substance 28 is a CAAC-OS film, a diffraction patternsimilar to that of an nc-OS film or the like is partly observed in somecases. Therefore, whether or not a CAAC-OS film is favorable can bedetermined by the proportion of a region where a diffraction pattern ofa CAAC-OS film is observed in a predetermined area (also referred to asproportion of CAAC). In the case of a high quality CAAC-OS film, forexample, the proportion of CAAC is higher than or equal to 50%,preferably higher than or equal to 80%, further preferably higher thanor equal to 90%, still further preferably higher than or equal to 95%.Note that a region where a diffraction pattern different from that of aCAAC-OS film is observed is referred to as the proportion of non-CAAC.

For example, transmission electron diffraction patterns were obtained byscanning a top surface of a sample including a CAAC-OS film obtainedjust after deposition (represented as “as-sputtered”) and a top surfaceof a sample including a CAAC-OS film subjected to heat treatment at 450°C. in an atmosphere containing oxygen. Here, the proportion of CAAC wasobtained in such a manner that diffraction patterns were observed byscanning for 60 seconds at a rate of 5 nm/second and the obtaineddiffraction patterns were converted into still images every 0.5 seconds.Note that as an electron beam, a nanobeam with a probe diameter of 1 nmwas used. The above measurement was performed on six samples. Theproportion of CAAC was calculated using the average value of the sixsamples.

FIG. 25A shows the proportion of CAAC of the samples. The proportion ofCAAC of the CAAC-OS film obtained just after the deposition was 75.7%(the proportion of non-CAAC was 24.3%). The proportion of CAAC of theCAAC-OS film subjected to the heat treatment at 450° C. was 85.3% (theproportion of non-CAAC was 14.7%). These results show that theproportion of CAAC obtained after the heat treatment at 450° C. ishigher than that obtained just after the deposition. That is, heattreatment at a high temperature (e.g., higher than or equal to 400° C.)reduces the proportion of non-CAAC (increases the proportion of CAAC).Further, the above results also indicate that even when the temperatureof the heat treatment is lower than 500° C., the CAAC-OS film can have ahigh proportion of CAAC.

Here, most of diffraction patterns different from that of a CAAC-OS filmare diffraction patterns similar to that of an nc-OS film. Further, anamorphous oxide semiconductor film was not able to be observed in themeasurement region. Therefore, the above results suggest that the regionhaving a structure similar to that of an nc-OS film is rearranged by theheat treatment owing to the influence of the structure of the adjacentregion, whereby the region becomes CAAC.

FIGS. 25B and 25C are planar TEM images of the CAAC-OS film obtainedjust after the deposition and the CAAC-OS film subjected to the heattreatment at 450° C., respectively. Comparison between FIGS. 25B and 25Cshows that the CAAC-OS film subjected to the heat treatment at 450° C.has more uniform film quality. That is, the heat treatment at a hightemperature improves the film quality of the CAAC-OS film.

With such a measurement method, the structure of an oxide semiconductorfilm having a plurality of structures can be analyzed in some cases.

Embodiment 4

In this embodiment, an example of a circuit including the transistor ofone embodiment of the present invention is described with reference tothe drawings.

FIG. 16A is a circuit diagram of a semiconductor device and FIGS. 16Cand 16D are each a cross-sectional view of a semiconductor device. FIGS.16C and 16D each illustrate a cross-sectional view of the transistor 100in a channel length direction on the left and a cross-sectional view ofthe transistor 100 in a channel width direction on the right. In thecircuit diagram, “OS” is written beside a transistor in order to clearlydemonstrate that the transistor uses an oxide semiconductor.

The semiconductor devices illustrated in FIGS. 16C and 16D each includea transistor 2200 using a first semiconductor material in a lowerportion and a transistor using a second semiconductor material in anupper portion. Here, an example is described in which the transistor 100described in Embodiment 1 as an example is used as the transistor usingthe second semiconductor material.

Note that FIGS. 17A and 17B illustrate examples of cross sections in thecase where the transistor 200 described in Embodiment 1 is used as thetransistor using the second semiconductor material.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, a semiconductor material other than an oxide semiconductor(e.g., silicon, germanium, silicon germanium, silicon carbide, orgallium arsenide) can be used as the first semiconductor material, andthe oxide semiconductor described in Embodiment 1 can be used as thesecond semiconductor material. A transistor using a material other thanan oxide semiconductor, such as single crystal silicon, can operate athigh speed easily. On the other hand, a transistor using an oxidesemiconductor has low off-state current.

Although the transistor 2200 is a p-channel transistor here, it isneedless to say that an n-channel transistor can be used to form acircuit having a different configuration. The specific structure of thesemiconductor device, such as a material used for the semiconductordevice and the structure of the semiconductor device, needs not to belimited to that described here except for the use of the transistordescribed in Embodiment 1, which is formed using an oxide semiconductor.

FIGS. 16A, 16C, and 16D each illustrate a configuration example of whatis called a CMOS circuit, in which a p-channel transistor and ann-channel transistor are connected in series and gates of thetransistors are connected.

The circuit can operate at high speed because the transistor of oneembodiment of the present invention that uses an oxide semiconductor hashigh on-state current.

FIG. 16C illustrates a configuration in which the transistor 100 isprovided over the transistor 2200 with an insulating layer 2201 providedtherebetween. A plurality of wirings 2202 are provided between thetransistor 2200 and the transistor 100. Furthermore, wirings andelectrodes provided over and under the insulating layers areelectrically connected to each other through a plurality of plugs 2203embedded in the insulating layers. Note that an insulating layer 2204covering the transistor 100, a wiring 2205 over the insulating layer2204, and a wiring 2206 formed by processing a conductive film that isalso used for the pair of electrodes of the transistor are provided.

When two transistors are stacked as described above, the area occupiedby the circuit can be reduced and a plurality of circuits can bearranged with higher density.

In FIG. 16C, one of a source and a drain of the transistor 100 iselectrically connected to one of a source and a drain of the transistor2200 through the wirings 2202 and the plugs 2203. The gate of thetransistor 100 is electrically connected to the gate of the transistor2200 through the wiring 2205, the wiring 2206, the plugs 2203, thewiring 2202, and the like.

In the configuration illustrated in FIG. 16D, an opening portion inwhich the plug 2203 is embedded is provided in the gate insulating layerof the transistor 100, and the gate of the transistor 100 is in contactwith the plug 2203. With such a configuration, the integration of thecircuit can be easily achieved and the lengths and the number of wiringsand plugs can be made smaller than those in the configurationillustrated in FIG. 16C; thus, the circuit can operate at higher speed.

Note that when a connection between the electrodes of the transistor 100and the transistor 2200 is changed from that in the configurationillustrated in FIG. 16C or FIG. 16D, a variety of circuits can beformed. For example, a circuit having a configuration in which a sourceand a drain of a transistor are connected to those of another transistoras illustrated in FIG. 16B can operate as what is called an analogswitch.

In addition, a semiconductor device having an image sensor function forreading data of an object can be manufactured with the use of any of thetransistors described in Embodiment 1 or 2.

FIG. 18 illustrates an example of an equivalent circuit of asemiconductor device having an image sensor function.

One electrode of a photodiode 602 is electrically connected to aphotodiode reset signal line 658, and the other electrode of thephotodiode 602 is electrically connected to a gate of a transistor 640.One of a source and a drain of the transistor 640 is electricallyconnected to a photosensor reference signal line 672, and the other ofthe source and the drain thereof is electrically connected to one of asource and a drain of a transistor 656. A gate of the transistor 656 iselectrically connected to a gate signal line 659, and the other of thesource and the drain thereof is electrically connected to a photosensoroutput signal line 671.

As the photodiode 602, for example, a pin photodiode in which asemiconductor layer having p-type conductivity, a high-resistancesemiconductor layer (semiconductor layer having i-type conductivity),and a semiconductor layer having n-type conductivity are stacked can beused.

With detection of light that enters the photodiode 602, data of anobject can be read. Note that a light source such as a backlight can beused at the time of reading data of an object.

Note that as each of the transistor 640 and the transistor 656, any ofthe transistors described as examples in Embodiment 1 or 2 in which achannel is formed in an oxide semiconductor can be used. In FIG. 18,“OS” is written beside each of the transistor 640 and the transistor 656so that it can be clearly identified that the transistors include anoxide semiconductor.

The transistor 640 and the transistor 656 are any of the transistorsdescribed as examples in the above embodiments, in which an oxidesemiconductor layer including a channel is wrapped with insulatinglayers each including an aluminum oxide film that contains excessoxygen. Furthermore, the transistor 640 and the transistor 656preferably have a structure in which the oxide semiconductor layer iselectrically surrounded by the gate electrode. The transistor 640 andthe transistor 656 are electrically stable transistors with less changein electrical characteristics. With the transistor, the semiconductordevice having an image sensor function, which is illustrated in FIG. 18,can be highly reliable.

This embodiment can be implemented in combination with any of the otherembodiments described in this specification as appropriate.

Embodiment 5

In this embodiment, an example of a semiconductor device (memory device)which uses a transistor of one embodiment of the present invention,which can hold stored data even when not powered, and which has anunlimited number of write cycles is described with reference todrawings.

FIG. 19 illustrates a circuit diagram of the semiconductor device.

The semiconductor device illustrated in FIG. 19 includes a transistor3200 using a first semiconductor material, a transistor 3300 using asecond semiconductor material, and a capacitor 3400. Note that as thetransistor 3300, any of the transistors described in the aboveembodiments can be used.

The transistor 3300 is a transistor in which a channel is formed in asemiconductor layer including an oxide semiconductor. Since theoff-state current of the transistor 3300 is small, stored data can beretained for a long period owing to such a transistor. In other words,power consumption can be sufficiently reduced because a semiconductormemory device in which refresh operation is unnecessary or the frequencyof refresh operation is extremely low can be provided.

In FIG. 19, a first wiring 3001 is electrically connected to a sourceelectrode of the transistor 3200. A second wiring 3002 is electricallyconnected to a drain electrode of the transistor 3200. A third wiring3003 is electrically connected to one of a source electrode and a drainelectrode of the transistor 3300. A fourth wiring 3004 is electricallyconnected to a gate electrode of the transistor 3300. A gate electrodeof the transistor 3200 and the other of the source electrode and thedrain electrode of the transistor 3300 are electrically connected to oneelectrode of the capacitor 3400. A fifth wiring 3005 is electricallyconnected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 19 utilizes a feature that thepotential of the gate electrode of the transistor 3200 can be held, andthus enables writing, storing, and reading of data as follows.

Writing and holding of data are described. First, the potential of thefourth wiring 3004 is set to a potential at which the transistor 3300 isturned on, so that the transistor 3300 is turned on. Accordingly, thepotential of the third wiring 3003 is applied to the gate electrode ofthe transistor 3200 and the capacitor 3400. That is, predeterminedcharge is given to the gate electrode of the transistor 3200 (writing).Here, charge for providing either of two different potential levels(hereinafter referred to as low-level charge and high-level charge) isapplied. Then, the potential of the fourth wiring 3004 is set to apotential at which the transistor 3300 is turned off, so that thetransistor 3300 is turned off. Thus, the charge given to the gateelectrode of the transistor 3200 is held (holding).

Since the off-state current of the transistor 3300 is extremely small,the charge of the gate electrode of the transistor 3200 is held for along time.

Next, reading of data is described. By applying an appropriate potential(a reading potential) to the fifth wiring 3005 while applying apredetermined potential (a constant potential) to the first wiring 3001,the potential of the second wiring 3002 varies depending on the amountof charge held in the gate electrode of the transistor 3200. This isbecause in general, when the transistor 3200 is an n-channel transistor,an apparent threshold voltage V_(th) _(_) _(H) in the case where ahigh-level charge is given to the gate electrode of the transistor 3200is lower than an apparent threshold voltage V_(th) _(_) _(L), in thecase where a low-level charge is given to the gate electrode of thetransistor 3200. Here, an apparent threshold voltage refers to thepotential of the fifth wiring 3005 which is needed to turn on thetransistor 3200. Thus, the potential of the fifth wiring 3005 is set toa potential V₀ which is between V_(th) _(_) _(H) and V_(th) _(_) _(L),whereby charge given to the gate electrode of the transistor 3200 can bedetermined. For example, in the case where the high-level charge isgiven in writing, when the potential of the fifth wiring 3005 is V₀(>V_(th) _(_) _(H)), the transistor 3200 is turned on. In the case wherethe low-level charge is given in writing, even when the potential of thefifth wiring 3005 is V₀ (<V_(th) _(_) _(L)), the transistor 3200 remainsoff. Therefore, the data held in the gate electrode can be read bydetermining the potential of the second wiring 3002.

Note that in the case where memory cells are arrayed to be used, onlydata of desired memory cells needs to be read. In the case where suchreading is not performed, a potential at which the transistor 3200 isoff regardless of the state of the gate electrode, that is, a potentialsmaller than V_(th) _(_) _(H) may be applied to the fifth wiring 3005.Alternatively, a potential at which the transistor 3200 is on regardlessof the state of the gate electrode, that is, a potential larger thanV_(th) _(_) _(L), may be applied to the fifth wiring 3005.

With the use of a transistor which includes a channel formation regionformed using an oxide semiconductor and has extremely small off-statecurrent, the semiconductor device in this embodiment can store data foran extremely long period. In other words, power consumption can besufficiently reduced because refresh operation becomes unnecessary orthe frequency of refresh operation can be extremely low. Moreover,stored data can be held for a long period even when power is notsupplied (note that a potential is preferably fixed).

Furthermore, in the semiconductor device described in this embodiment,high voltage is not needed for writing data and there is no problem ofdeterioration of elements. For example, unlike a conventionalnonvolatile memory, it is not necessary to inject and extract electronsinto and from a floating gate, and thus a problem such as deteriorationof a gate insulating layer does not arise. In other words, thesemiconductor device of one embodiment of the present invention does nothave a limit on the number of times of writing which is a problem in theconventional nonvolatile memory, and reliability thereof is drasticallyimproved. Moreover, data is written depending on the on state and theoff state of the transistor, whereby high-speed operation can be easilyachieved.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 6

In this embodiment, description is given of a CPU in which at least thetransistor described in any of the above embodiments can be used and thememory device described in Embodiment 5 is included.

FIG. 20 is a block diagram illustrating an example of the configurationof a CPU at least partly including any of the transistors described inEmbodiment 1.

The CPU illustrated in FIG. 20 includes an arithmetic logic unit (ALU)1191, an ALU controller 1192, an instruction decoder 1193, an interruptcontroller 1194, a timing controller 1195, a register 1196, a registercontroller 1197, a bus interface (Bus I/F) 1198, a rewritable ROM 1199,and an ROM interface (ROM I/F) 1189 over a substrate 1190. Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The ROM 1199 and the ROM interface1189 may be provided over a separate chip. Needless to say, the CPU inFIG. 20 is just an example of a simplified configuration, and an actualCPU may have a variety of configurations depending on the application.For example, the CPU may have a configuration including a plurality ofcores that operate in parallel; each of the cores has a structureincluding the CPU or the arithmetic circuit illustrated in FIG. 20. Thenumber of bits that the CPU can process in an internal arithmeticcircuit or in a data bus can be 8, 16, 32, or 64, for example.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 processes an interrupt request from an external input/output deviceor a peripheral circuit depending on its priority or a mask state. Theregister controller 1197 generates an address of the register 1196, andreads/writes data from/to the register 1196 depending on the state ofthe CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 on the basis of areference clock signal CLK1, and supplies the internal clock signal CLK2to the above circuits.

In the CPU illustrated in FIG. 20, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of thetransistors described in the above embodiments can be used.

In the CPU in FIG. 20, the register controller 1197 selects an operationof holding data in the register 1196, in response to an instruction fromthe ALU 1191. That is, the register controller 1197 selects whether datais held by a flip-flop or by a capacitor in the memory cell included inthe register 1196. When data holding by the flip-flop is selected, powersupply voltage is supplied to the memory cell in the register 1196. Whendata holding by the capacitor is selected, the data is rewritten in thecapacitor, and supply of power supply voltage to the memory cell in theregister 1196 can be stopped.

FIG. 21 is an example of a circuit diagram of a memory element that canbe used as the register 1196. A memory element 700 includes a circuit701 in which stored data is volatile when power supply is stopped, acircuit 702 in which stored data is nonvolatile when power supply isstopped, a switch 703, a switch 704, a logic element 706, a capacitor707, and a circuit 720 having a selecting function. The circuit 702includes a capacitor 708, a transistor 709, and a transistor 710. Notethat the memory element 700 may further include another element such asa diode, a resistor, or an inductor, as needed.

Here, the memory device described in Embodiment 5 can be used as thecircuit 702. When supply of the power supply voltage to the memoryelement 700 is stopped, a ground potential (0 V) or a potential at whichthe transistor 709 is turned off continues to be input to a gate of thetransistor 709 in the circuit 702. For example, the gate of thetransistor 709 is grounded through a load such as a resistor.

An example in which the switch 703 is a transistor 713 having oneconductivity type (e.g., an n-channel transistor) and the switch 704 isa transistor 714 having a conductivity type opposite to the oneconductivity type (e.g., a p-channel transistor) is described. Here, afirst terminal of the switch 703 corresponds to one of a source and adrain of the transistor 713, a second terminal of the switch 703corresponds to the other of the source and the drain of the transistor713, and conduction or non-conduction between the first terminal and thesecond terminal of the switch 703 (i.e., the on/off state of thetransistor 713) is selected by a control signal RD input to a gate ofthe transistor 713. A first terminal of the switch 704 corresponds toone of a source and a drain of the transistor 714, a second terminal ofthe switch 704 corresponds to the other of the source and the drain ofthe transistor 714, and conduction or non-conduction between the firstterminal and the second terminal of the switch 704 (i.e., the on/offstate of the transistor 714) is selected by the control signal RD inputto a gate of the transistor 714.

One of a source and a drain of the transistor 709 is electricallyconnected to one of a pair of electrodes of the capacitor 708 and a gateof the transistor 710. Here, the connection portion is referred to as anode M2. One of a source and a drain of the transistor 710 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 703 (the one of the sourceand the drain of the transistor 713). The second terminal of the switch703 (the other of the source and the drain of the transistor 713) iselectrically connected to the first terminal of the switch 704 (the oneof the source and the drain of the transistor 714). The second terminalof the switch 704 (the other of the source and the drain of thetransistor 714) is electrically connected to a line which can supply apower supply potential VDD. The second terminal of the switch 703 (theother of the source and the drain of the transistor 713), the firstterminal of the switch 704 (the one of the source and the drain of thetransistor 714), an input terminal of the logic element 706, and one ofa pair of electrodes of the capacitor 707 are electrically connected toeach other. Here, the connection portion is referred to as a node M1.The other of the pair of electrodes of the capacitor 707 can be suppliedwith a constant potential. For example, the other of the pair ofelectrodes of the capacitor 707 can be supplied with a low power supplypotential (e.g., GND) or a high power supply potential (e.g., VDD). Theother of the pair of electrodes of the capacitor 707 is electricallyconnected to the line which can supply a low power supply potential(e.g., a GND line). The other of the pair of electrodes of the capacitor708 can be supplied with a constant potential. For example, the other ofthe pair of electrodes of the capacitor 708 can be supplied with a lowpower supply potential (e.g., GND) or a high power supply potential(e.g., VDD). The other of the pair of electrodes of the capacitor 708 iselectrically connected to the line which can supply a low power supplypotential (e.g., a GND line).

The capacitor 707 and the capacitor 708 are not necessarily provided aslong as the parasitic capacitance of the transistor, the wiring, or thelike is actively utilized.

A control signal WE is input to a first gate (first gate electrode) ofthe transistor 709. As for each of the switch 703 and the switch 704, aconduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD which isdifferent from the control signal WE. When the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

A signal corresponding to data held in the circuit 701 is input to theother of the source and the drain of the transistor 709. FIG. 21illustrates an example in which a signal output from the circuit 701 isinput to the other of the source and the drain of the transistor 709.The logic value of a signal output from the second terminal of theswitch 703 (the other of the source and the drain of the transistor 713)is inverted by the logic element 706, and the inverted signal is inputto the circuit 701 through the circuit 720.

In the example of FIG. 21, a signal output from the second terminal ofthe switch 703 (the other of the source and the drain of the transistor713) is input to the circuit 701 through the logic element 706 and thecircuit 720; however, this embodiment is not limited thereto. The signaloutput from the second terminal of the switch 703 (the other of thesource and the drain of the transistor 713) may be input to the circuit701 without its logic value being inverted. For example, in the casewhere a node in which a signal obtained by inversion of the logic valueof a signal input from the input terminal is held is provided in thecircuit 701, the signal output from the second terminal of the switch703 (the other of the source and the drain of the transistor 713) can beinput to the node.

As the transistor 709 in FIG. 21, any of the transistors described inEmbodiment 1 can be used. The transistor 709 preferably includes asecond gate (second gate electrode) that faces the first gate with asemiconductor layer provided therebetween. The control signal WE can beinput to the first gate and the control signal WE2 can be input to thesecond gate. The control signal WE2 is a signal having a constantpotential. As the constant potential, for example, a ground potentialGND or a potential lower than a source potential of the transistor 709is selected. The control signal WE2 is a potential signal forcontrolling the threshold voltage of the transistor 709, and the cut-offcurrent (Icut) of the transistor 709 can be further reduced. Note thatas the transistor 709, the transistor without the second gate can beused.

Furthermore, in FIG. 21, the transistors included in the memory element700 except for the transistor 709 can each be a transistor in which achannel is formed in a layer formed using a semiconductor other than anoxide semiconductor or in the substrate 1190. For example, thetransistor can be a transistor in which a channel is formed in a siliconlayer or a silicon substrate. Alternatively, a transistor in which achannel is formed in an oxide semiconductor layer can be used for allthe transistors used for the memory element 700. Further alternatively,in the memory element 700, a transistor in which a channel is formed inan oxide semiconductor layer can be included besides the transistor 709,and a transistor in which a channel is formed in a layer or thesubstrate 1190 including a semiconductor other than an oxidesemiconductor can be used for the rest of the transistors.

As the circuit 701 in FIG. 21, for example, a flip-flop circuit can beused. As the logic element 706, for example, an inverter, a clockedinverter, or the like can be used.

In the semiconductor device of one embodiment of the present invention,in a period during which the memory element 700 is not supplied with thepower supply voltage, data stored in the circuit 701 can be held by thecapacitor 708 which is provided in the circuit 702.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor layer is extremely low. For example, the off-statecurrent of a transistor whose channel is formed in an oxidesemiconductor layer is much lower than that of a transistor whosechannel is formed in crystalline silicon. Thus, when such a transistorincluding an oxide semiconductor is used for the transistor 709, asignal held in the capacitor 708 is held for a long time also in aperiod during which the power supply voltage is not supplied to thememory element 700. The memory element 700 can accordingly hold thestored content (data) also in a period during which the supply of thepower supply voltage is stopped.

Since the switch 703 and the switch 704 are provided, the memory elementperforms pre-charge operation; thus, the time required for the circuit701 to hold original data again after the supply of the power supplyvoltage is restarted can be shortened.

In the circuit 702, a signal held by the capacitor 708 is input to thegate of the transistor 710. Therefore, after supply of the power supplyvoltage to the memory element 700 is restarted, the signal held by thecapacitor 708 can be converted into the one corresponding to the state(the on state or the off state) of the transistor 710 to be read fromthe circuit 702. Consequently, an original signal can be accurately readeven when a potential corresponding to the signal held by the capacitor708 fluctuates to some degree.

By using the above-described memory element 700 in a memory device suchas a register or a cache memory included in a processor, data in thememory device can be prevented from being lost owing to the stop of thesupply of the power supply voltage. Further, shortly after the supply ofthe power supply voltage is restarted, the memory element can bereturned to the same state as that before the power supply is stopped.Therefore, the power supply can be stopped even for a short time in theprocessor or one or a plurality of logic circuits included in theprocessor. Accordingly, power consumption can be reduced.

Although an example in which the memory element 700 is used in a CPU isdescribed in this embodiment, the memory element 700 can also be used ina digital signal processor (DSP), a custom LSI, an LSI such as aprogrammable logic device (PLD), and a radio frequency identification(RF-ID).

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 7

In this embodiment, description is given of examples of an electronicdevice which can include any of the semiconductor devices described inthe above embodiments, such as the transistors, the memory device, andthe CPU (including a DSP, a custom LSI, a PLD, and an RF-ID).

Any of the transistors, the memory device, and the CPU described in theabove embodiments can be used in a variety of electronic devices(including game machines). Examples of the electronic devices includedisplay devices of televisions, monitors, and the like, lightingdevices, personal computers, word processors, image reproductiondevices, portable audio players, radios, tape recorders, stereos,phones, cordless phones, mobile phones, car phones, transceivers,wireless devices, game machines, calculators, portable informationterminals, electronic notebooks, e-book readers, electronic translators,audio input devices, video cameras, digital still cameras, electricshavers, IC chips, high-frequency heating appliances such as microwaveovens, electric rice cookers, electric washing machines, electric vacuumcleaners, air-conditioning systems such as air conditioners,dishwashers, dish dryers, clothes dryers, futon dryers, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, radiation counters, and medical equipmentsuch as dialyzers and X-ray diagnostic equipment. The examples of theelectronic devices further include alarm devices such as smokedetectors, heat detectors, gas alarm devices, and security alarmdevices. In addition, industrial equipment such as guide lights, trafficlights, belt conveyors, elevators, escalators, industrial robots, andpower storage systems can be included in the examples. Furthermore,moving objects and the like driven by fuel engines and electric motorsusing power from non-aqueous secondary batteries are also included inthe category of electronic devices. Examples of the moving objectsinclude electric vehicles (EV), hybrid electric vehicles (HEV) whichinclude both an internal-combustion engine and a motor, plug-in hybridelectric vehicles (PHEV), tracked vehicles in which caterpillar tracksare substituted for wheels of these vehicles, motorized bicyclesincluding motor-assisted bicycles, motorcycles, electric wheelchairs,golf carts, boats or ships, submarines, helicopters, aircrafts, rockets,artificial satellites, space probes, planetary probes, and spacecrafts.Some specific examples of these electronic devices are illustrated inFIGS. 22A to 22C.

In a television set 8000 illustrated in FIG. 22A, a display portion 8002is incorporated in a housing 8001. The display portion 8002 can displayan image and a speaker portion 8003 can output sound. Any of thetransistors described in the above embodiments can be used in a pixel ora driver circuit for operating the display portion 8002 incorporated inthe housing 8001.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoretic displaydevice, a digital micromirror device (DMD), or a plasma display panel(PDP) can be used for the display portion 8002.

The television set 8000 may be provided with a receiver, a modem, andthe like. With the receiver, a general television broadcast can bereceived. Furthermore, when the television set 8000 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver, between receivers, or the like) datacommunication can be performed.

In addition, the television set 8000 may include a CPU 8004 forperforming information communication or a memory. Any of thetransistors, the memory device, and the CPU described in the aboveembodiments is used for the CPU 8004 or the memory, whereby powerconsumption can be reduced.

An alarm device 8100 illustrated in FIG. 22A is a residential firealarm, which is an example of an electronic device including a sensorportion 8102 for smoke or heat and a microcomputer 8101. Themicrocomputer 8101 includes any of the transistors, the memory device,and the CPU described in the above embodiments.

An air conditioner which includes an indoor unit 8200 and an outdoorunit 8204 illustrated in FIG. 22A is an example of an electronic deviceincluding any of the transistors, the memory device, the CPU, and thelike described in the above embodiments. Specifically, the indoor unit8200 includes a housing 8201, an air outlet 8202, a CPU 8203, and thelike. Although the CPU 8203 is provided in the indoor unit 8200 in FIG.22A, the CPU 8203 may be provided in the outdoor unit 8204.Alternatively, the CPU 8203 may be provided in both the indoor unit 8200and the outdoor unit 8204. Any of the transistors described in the aboveembodiments is used in the CPU in the air conditioner, whereby powerconsumption can be reduced.

An electronic refrigerator-freezer 8300 illustrated in FIG. 22A is anexample of an electronic device including any of the transistors, thememory device, the CPU, and the like described in the above embodiments.Specifically, the electric refrigerator-freezer 8300 includes a housing8301, a door for a refrigerator 8302, a door for a freezer 8303, a CPU8304, and the like. In FIG. 22A, the CPU 8304 is provided in the housing8301. Any of the transistors described in the above embodiments is usedin the CPU 8304 of the electric refrigerator-freezer 8300, whereby powerconsumption can be reduced.

FIGS. 22B and 22C illustrate an example of an electric vehicle which isan example of an electronic device. An electric vehicle 9700 is equippedwith a secondary battery 9701. The output of the electric power of thesecondary battery 9701 is adjusted by a circuit 9702 and the electricpower is supplied to a driving device 9703. The circuit 9702 iscontrolled by a processing unit 9704 including a ROM, a RAM, a CPU, orthe like which is not illustrated. Any of the transistors described inthe above embodiments is used in the CPU in the electric vehicle 9700,whereby power consumption can be reduced.

The driving device 9703 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 9704 outputs a control signal to the circuit 9702 based on inputdata such as data of operation (e.g., acceleration, deceleration, orstop) by a driver or data during driving (e.g., data on an upgrade or adowngrade, or data on a load on a driving wheel) of the electric vehicle9700. The control circuit 9702 adjusts the electric energy supplied fromthe secondary battery 9701 in accordance with the control signal of theprocessing unit 9704 to control the output of the driving device 9703.In the case where the AC motor is mounted, although not illustrated, aninverter which converts direct current into alternate current is alsoincorporated.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

This application is based on Japanese Patent Application serial no.2013-106223 filed with Japan Patent Office on May 20, 2013 and JapanesePatent Application serial no. 2013-106253 filed with Japan Patent Officeon May 20, 2013, the entire contents of which are hereby incorporated byreference.

What is claimed is:
 1. A semiconductor device comprising: a firstinsulating layer comprising aluminum oxide; an oxide semiconductor layerover the first insulating layer; a source electrode and a drainelectrode electrically connected to the oxide semiconductor layer; agate insulating layer over the source electrode and the drain electrode,a gate electrode overlapping with the oxide semiconductor layer with thegate insulating layer provided therebetween; and a second insulatinglayer comprising aluminum oxide over the gate electrode, wherein thefirst insulating layer and the second insulating layer are in contactwith each other in a region where the source electrode, the drainelectrode, and the gate electrode are not provided, and wherein thefirst insulating layer comprises a protrusion.
 2. The semiconductordevice according to claim 1, wherein the source electrode and the drainelectrode are provided over the oxide semiconductor layer.
 3. Thesemiconductor device according to claim 1, wherein the first insulatinglayer is in contact with the oxide semiconductor layer.
 4. Thesemiconductor device according to claim 1, wherein the second insulatinglayer is in contact with the gate electrode.
 5. The semiconductor deviceaccording to claim 1, wherein a thickness of the oxide semiconductorlayer is 0.1 times or more and 10 times or less as large as a channelwidth of the oxide semiconductor layer.
 6. The semiconductor deviceaccording to claim 1, wherein the oxide semiconductor layer comprisesindium.
 7. A semiconductor device comprising: a first insulating layercomprising aluminum oxide; an oxide semiconductor layer over the firstinsulating layer; a source electrode and a drain electrode electricallyconnected to the oxide semiconductor layer; a gate insulating layer overthe source electrode and the drain electrode, a gate electrode over thegate insulating layer, the gate electrode overlapping with the oxidesemiconductor layer; and a second insulating layer comprising aluminumoxide over the gate electrode, wherein the first insulating layer andthe second insulating layer are in contact with each other in a regionwhere the source electrode, the drain electrode, and the gate electrodeare not provided.
 8. The semiconductor device according to claim 7,wherein the source electrode and the drain electrode are provided overthe oxide semiconductor layer.
 9. The semiconductor device according toclaim 7, wherein the first insulating layer is in contact with the oxidesemiconductor layer.
 10. The semiconductor device according to claim 7,wherein the second insulating layer is in contact with the gateelectrode.
 11. The semiconductor device according to claim 7, wherein athickness of the oxide semiconductor layer is 0.1 times or more and 10times or less as large as a channel width of the oxide semiconductorlayer.
 12. The semiconductor device according to claim 7, wherein theoxide semiconductor layer comprises indium.
 13. A semiconductor devicecomprising: a first insulating layer comprising aluminum oxide; a firstoxide layer over the first insulating layer; an oxide semiconductorlayer over the first oxide layer; a second oxide layer over the oxidesemiconductor layer; a source electrode and a drain electrodeelectrically connected to the oxide semiconductor layer; a gateinsulating layer over the source electrode and the drain electrode, agate electrode over the gate insulating layer, the gate electrodeoverlapping with the oxide semiconductor layer; and a second insulatinglayer comprising aluminum oxide over the gate electrode, wherein thefirst insulating layer and the second insulating layer are in contactwith each other in a region where the source electrode, the drainelectrode, and the gate electrode are not provided.
 14. Thesemiconductor device according to claim 13, wherein the source electrodeand the drain electrode are provided over the oxide semiconductor layer.15. The semiconductor device according to claim 13, wherein the firstinsulating layer is in contact with the first oxide layer.
 16. Thesemiconductor device according to claim 13, wherein the secondinsulating layer is in contact with the gate electrode.
 17. Thesemiconductor device according to claim 13, wherein a thickness of theoxide semiconductor layer is 0.1 times or more and 10 times or less aslarge as a channel width of the oxide semiconductor layer.
 18. Thesemiconductor device according to claim 13, wherein the oxidesemiconductor layer comprises indium.